My sewing background is fairly extensive and I spent four years working in theater costume construction, so I may have some techniques that you're not familiar with even if you're a handy home sewer. Give them a try. I also have a small library of great reference books which I might mention throughout the instructable.
Step 1: Find Inspiration
First, they show you the pure style idea with no distractions and often in several variations.
Second, the illustrations tend to show seam lines and darts, which makes it easier for me to draft the pattern myself if I want to.
Third, and possibly most important, the back of the envelope has yardage indications for each size and for various widths of fabric. Very, very useful.
You can also look at finished garments, or dream up something in your own imagination.
I came up with this sketch after I'd seen the fabric I knew I wanted to do something with. I made a couple of changes during construction (I made an attached collar rather than two separate ones, and there's no bust dart), but otherwise the finished shirt is corresponds pretty much to the design and I was very excited with it.
Step 2: Gather Materials
1. A piece of fabric
I bought my main fabric on Etsy.com in their vintage/supplies/fabric section. eBay is also a good source. I only sew with natural fibers because they're more comfortable, so I won't buy poly blends or nylon/dacron - check the description. Vintage fabric is narrower than most modern fabric, so keep this in mind when you're shopping. My two-yard piece is enough for a collared shirt with elbow-length sleeves for my 34" bust figure. One yard of 35" fabric is not really enough to make anything, except maybe a cocktail apron.
I also bought half a yard of contrast fabric from my local fabric store for the cuffs, collar stand, and buttons.
Don't forget matching thread. Sew-all or all-purpose style threads are fine.
Vintage buttons are cheap and readily available - again, Etsy is a good source. I love covered buttons, so that's what I'm going with here. If you're new to covered buttons, make sure the first package you buy comes with the little tool for covering.
I don't make a muslin mock-up in this instructable because the pattern is from a shirt that I already know fits me, but every time I make something from a new pattern or a pattern that I drafted myself, I always make a muslin test garment first. Muslin is cheap and in endless supply. Your vintage fabric is one-of-a-kind. You can also use muslin as interfacing.
The only kind of interfacing I really don't like is the non-woven stuff. For this shirt I'm interfacing with regular cotton fabric from scraps of my shirt material, but I also have woven fusible interfacing in my sewing room and that's nice to work with as well.
6. Bias tape
I use some double fold bias tape for the armscye seam of this shirt, but it's not absolutely necessary.
7. A pattern or a well-fitting shirt you're willing to slice up (just cut carefully along the seam lines)
You will also need a few tools.
A sewing machine is useful (but not necessary if you have a steady hand and lots of time).
An iron is indispensable. If your ironing board has a sleeve board, great; a tailor's sausage is the next best thing.
A measuring tape comes in handy if you're doing any pattern alterations.
Good fabric shears make life much better, and they're not prohibitively expensive.
Depending on your fabric, either tailor's wax chalk or "tracing paper" (paper with waxy, colored stuff on one side) and a tracing wheel will give you accurate pattern pieces. Both have their advantages. Tailor's wax chalk disappears when you steam/iron it, which is convenient until you're pressing a seam open and you accidentally erase sewing lines you haven't used yet. On the other hand, the marks from tracing paper can be difficult to get out of your fabric, and on fine materials they often show through to the right side.
Step 3: Prep Materials
You'll need to shrink your material before working with it. For cotton, this means a trip through the washer and dryer on the hottest settings you'll ever use - you want all shrinkage to happen now, before you cut and sew. The fabric should get the roughest treatment it's likely to get in its lifetime. For other materials, like silk and wool, that will not endure machine treatment, the "damp roll" is my method of choice: run a towel through the wash and spin cycle, then roll up the dry fabric in the wet towel. Let it sit overnight. Then unroll and air dry the fabric.
Following shrinking, press or iron your fabric, taking care to straighten the grain as you go so your fabric is as straight and true as possible. My fabric is a plissé, so it won't be flat even after ironing, but it will be straight and on-grain. ("Grain" refers to the perpendicular crossed threads - the long threads represent the straight grain, and the shorter threads - from selvage to selvage, the finished edges of the material - the cross grain.)
Your fabric is now prepped for layout and cutting.
Step 4: Pattern Lay Out
I had a shirt, the fit of which I liked, but the material of which left me indifferent. I cut the pieces apart and ripped out the dart seams, tracing the dart with pen before ironing the pieces flat. I also labeled them (always a good idea). The changes I wanted to make included giving myself a little more room in the bust (+2"), adding a little 1940's puff to the sleeve head and shortening the sleeve to elbow length, and changing the pointed collar to a Peter Pan style, to which I'll add a little tie. These changes are minor enough that I will not make a new pattern, but simply make the changes on the new fabric.
Lay out your fabric with right sides together or down. I usually fold my fabric in half with the fold on the straight grain and then lay out pieces that I need two of (like the front and the sleeve). The back, which is one piece, I place on the fold. The rest I take care of with whatever's left. If your yardage is tight, though, lay everything out first before cutting anything.
Once your pieces are where you want them, trace them (again, make sure your tracing lines are not on the fabric's right side). With my vintage print, I had to use tracing paper because my tailor's chalk wasn't showing very well. I used the chalk on my solid navy contrast fabric, though. These lines are my sewing lines, not my cutting lines, so when I cut the pieces, I leave space (I like a half inch to 3/4 of an inch) around the lines.
Step 5: Construction
My shirt has a yoke, so I start there.
1. Yoke pieces to shirt back. Grade the seams with the widest seam allowance to the outside. Optional: edge stitch the outer yoke piece and the seam allowances (not the inner piece). Edge stitching has to be precise to look good, and on my plissé fabric it just looked messy, so I opted out of that step.
2. Front edges: the fronts of a button-down shirt have an extra 2" or 1 3/4" and they get folded twice (with the raw edge completely enclosed) to make a 1" wide edge that supports the buttons and buttonholes. This will be part of your pattern. It's generally a good idea to interface the buttonhole side. I used a strip of cotton (from my contrast piece) a little shy of an inch wide buried under the folded edge. You can fold the edges in or out; I like the look of the outside fold, so I fold the buttonhole side out. Edge stitch or top stitch both the left and right sides of the fold.
3. Front and yoke shoulder seams: with the wrong sides of the fronts and back together, stitch the front shoulders to the shoulders of the inside yoke piece.
4. Now the outer yoke at the top of the shoulders will be open. Spread the shirt flat, press the yoke area, and then fold the shoulder seam allowance to the inside. Press it again and edge stitch the shoulder seams. The yoke and shoulders are now finished.
5. If you have a little sleeve puff going on, stitch two rows of stitches in the seam allowance of your sleeve heads, using the longest stitch length you have. Then, pin the sleeves to the armscye of the shirt (this is different from constructing a jacket or other garment where you sew the sleeve and side seams first, and then fit the finished sleeve into the hole). Draw your gathering rows up until you have the gathers where you want. Stitch. If you're felling the armscye seam, do it now. (I don't, because mine is gathered; after step 6, I use bias tape to finish the seam allowances.)
6. Stitch the sleeves and the sides as one seam (but don't sew the seam allowances down - stop and secure the stitching on either side). Fell the seams, if you like, or finish them with a serger or seam binding or whatever you prefer. You now have a shirt, so take a break, try it on, and pat yourself on the back.
Step 6: Finishing, Part I: Collar and Cuffs
My cuffs have no buttons or anything to make them complicated. I sewed the outer edges and then turned and pressed them, then sewed the outside edge to the sleeve. To sew the inside edge, you'll "stitch in the ditch" which, for me, is a little tricky. You've got to pin the inside edge, folded under, just barely deeper than the outside seam, and then sew from the outside right in the "ditch" between the two pieces of fabric, essentially along the same exact seam you just made. This is invisible from the outside when you do it well. With the cuff edges, do whatever is neat and easy. If you've pressed ahead of time, you can pretty much just play with the folds until they hide themselves, and then sew a single line of stitching to keep everything in place. I added a little piece of elastic inside to keep my cuffs from slipping down past my elbows.
The collar is trickier. Again, I referred to Shirt Making for comprehensive directions. The collar stand goes on first, one piece at a time: first the outside piece, then the inside piece. Line the pieces up and pin them, then sew them, from the center back seam out toward the front edges. When you're sewing the inside piece, pin it in place from the outside and sew with the inside piece on the bottom so that you can sew along the same line of stitching that's on the outside piece. Don't sew past the front edges of the shirt.
Make the collar. A piece of interfacing inside makes for a crisper, neater finish. Sew all pieces together, leaving the straight bottom edge open, and turn. Press the collar, pulling the underside and stretching the top a little so that the seam lies underneath rather than right on the edge. Top stitch the collar a quarter inch from the edge, paying attention to that seam and making sure you keep it shifted underneath.
Now hold the collar up to the stand and mark where the collar ends. You'll need to sew the collar stand up to those points, which means rolling the shirt fronts out of the way and sort of inside the collar stand pieces. It's not too bad, because you only need to get to the corner and sew the stand shut up to the point you just marked. Sew to that point, trim and clip all seam allowances inside the stand, and then turn and press.
Now just slip the collar into the open top of the stand, carefully fold the edges of the stand under, pin them together, and edge stitch around the stand from the top side. Press.
Step 7: Finishing, Part II: Buttons and Hem
Make buttonholes in your manner of choosing. I'm making them by hand (refer to Cabera's Classic Tailoring Techniques for great directions on handworked buttonholes), but you can use your machine - just make a few practice buttonholes first. Buttonholes should be appropriately sized for the buttons you've chosen.
Line up the shirt fronts again flat on a table and mark with chalk or a pin through the buttonholes onto the other front edge. Sew your buttons on. Make a shank if you have the kind of buttons with holes in them (common for shirt buttons).
Finally, hem the shirt. Though some instructions suggest doing this earlier, I like to hem as the last step on pretty much everything I do. With all buttons buttoned, make sure your edges line up and trim away anything extra. Either make a rolled hem with a rolled hem foot or by hand, or use some single fold bias tape to tape up the edge. I went with the hand-rolled hem, because the plissé stretched too much under my rolled hem foot. I always trust my hands more than my machine when it comes to anything tricky, even though I love my machine. To make the hem by hand, first pin it carefully with lots of pins, then baste it by hand and make sure it's how you want it - lies flat, even on both sides, and so on. Don't skip the basting on these tiny, curved hems. It's worth the trouble for the better result. Machine-stitch, and finally remove the basting. Press.
Look over the shirt and trim any thread tails hanging around, check the undersides of topstitched and edge stitched seams to make sure everything's secure, and generally give it a once-over. If everything looks good, put it on and prepare for compliments. Now I need to make a skirt out of that navy cotton to go with the blouse.