We recently acquired a vintage sewing machine desk and decided we wanted to convert it to being a seamstess desk/computer desk! This instructable will show some tips on how to customize your furniture for your needs!

Needed:

Vintage Sewing Desk with sewing machine removed
12" x 6' x .75" plank of poplar (or wood of choice)

Tools Needed:

Table saw
Band saw
Router
Mouse Sander
Wood stains (We used Minwax Golden Mahogany and Chinese calligraphy ink)
Minwax Paste Finishing Wax
Shop Towels, lots of shop towels
Square
Screws/Screwdriver
Center-punch/Hammer
Chisels
Tape Measure/Painter's tape
Clamps (box clamps / band clamps)
Wood glue
Pencil!

Step 1: Get Your Desk Measurements

Welcome to our desk that we're about to customize! For this particular desk, we want to eventually use it as a computer desk and know that we want to add a shelf that our keyboard can fit under. We also want to add a wing to the side of the desk - for the mouse, or whatever other items we end up putting on there!

Measure your desk to see how wide your shelf needs to be. For our purposes, the shelf sides needed to be about 4" tall.
Ensure that the piece of wood you're using is long enough, and as long as it is, you can safely mark out where your cuts need to be. Our piece of wood had a neat spalted/flame pattern on it, so we adjusted where our marks were to get the most out of the flame grain.
Also note, we wanted to have the flame wrap sequentially from one side to the other. So we made our cuts in a sequential fashion and kept track of which piece was next to the other.

Step 2: Cut the Dados for the Shelf Sides

There are many different types of joints that could have been used, and we chose the dado joint for the shelf legs.

First examine to see how the shelf fits on the desk and measure/mark where the dados need to be cut.
Using your table saw, set your saw blade height to be half of what the plank's width is. In our case, it was set to 3/8" because the plank's width is 3/4".
The cleanest way we came up with for cutting these dados was to cut the outside borders of the dado first before getting rid of the inside wood. It will go one slice at a time and it will take patience! You could also do this with a router.
Do this on both sides!
Check the fit to make sure that the shelf sides will fit into the dado joints. If necessary, make a few more cuts.

Step 3: Clean Up the Dados and Glue Up!

We don't have a dado blade to make the cuts, and as a result, there are some very small grooves that have resulted post-cuts. After doing the test-fits to make sure the shelf sides could fit into the dado joint, we used a chisel to clean up the grooves.

Do this for both sides! Put the shelf together to make sure it still looks good! Then, using your clamps and wood glue, glue up the shelf.

Step 4: Measure and Cut the Wing and Its Sides

On the side of the desk, we are also going to add a wing with braces. While the shelf is curing, measure out how big you need the wing to be. We wanted our's to fit an acoustic coupler so that's what we used as a reference.

For the wing braces, we cut out two pieces that were the same in size. Since they are supposed to be matching, we made sure to tape them together before cutting them. These were the same width as the wing.
For the wing braces, we wanted to take the curve of the existing desk's curves, and bring that into the brace design. So using one of our french curves, we found the closest one available on the desk and drew that onto the taped blocks.
For ease of mounting, be sure to stop the curve approximately 1" before the end of the block.

Once you've drawn out your lines, use a bandsaw and carefully cut out the braces! And of course, sand out any imperfections from the cutting process.

Step 5: Bring Your Shelf Back in Time

Now that the shelf has finished curing, it's time to bring it into the same decade as the rest of the desk. We noticed that the edge of the table was routed. Luckily, we have a router bit which is capable of giving us a similarly routed edge!

So we routed the sides of the shelf top and using various grits of sandpaper, got it all smooth and prepped for finishing.

Step 6: Put the Stain on the Shelf

Since the wood we are using is poplar, getting a stain on it to look even and uniform can be quite difficult. There are a few ways to minimize drastic stain blotching and this is one of the techniques.

Use acetone to wet the wood. This raises the poplar's grain just enough to be slightly fuzzy.
Sand this extra fuzz off before it has time to rest back down into the wood. This will give you a super smooth surface.
Put a layer of linseed oil on. This will soak into the wood and create a buffer that your stain ideally will not go past - and therefore allow you to have a uniform stain layer. Allow this to "cure" for 15 minutes or so.
Once the linseed oil has soaked in a bit, you can begin putting on your stain. We used Minwax Golden Mahogany stain, which in our tests, seemed to match most with the existing wood color.
We were generous with the stain and tried to push as much stain into the wood as possible. Once it got to the point we were satisfied with, we wiped off any excess and let it cure for 30 minutes or so.

Step 7: Waxing the Shelf

Now that the color is where you want it, it's time to add some finishing touches! We use Minwax Finishing Paste and have found that the best way to apply it is by hand. It comes out as a gritty paste, but with the heat of your hands, the wax melts quite nicely.

After applying it to the shelf, we let it cure for 15 minutes. Once you see white specks, you'll know it's ready to be "scraped" and buffed. Use a scrap piece of wood to gently scrap off any excess wax spots.

Once the excess wax has been scrapped off, use a cloth to first hand buff the wax, and then for a super shine, use a mouse sander on a shop cloth or normal cloth to get an amazing result!

Step 8: Add Rabbet Joints to the Wing Braces

We haven't forgotten the wing or its braces yet!

We decided that the wing would sit in rabbet joints on the two braces. The rabbet joints will be cut so that the wing is flush with the tops of the braces. Also, the width of the joint would be half the width of the board thickness - so 3/8" wide, 3/4" deep cuts need to be made into the braces!
There are a couple different ways to do this. You could cut the board a slice at a time like what was done with the dado joints, or you could just cut the borders as depicted in the pictures!
Do this for both sides, but remember, the wing is sitting between the joints.

Step 9: Test the Fit and Adjust the Wing If Necessary

Once cutting the rabbet joints, check for the fit of the board. You may need to cut a few times more to get the board to fit perfectly.
Once it's in a good spot, glue the braces and wing together! Brace carefully! Allow this to cure for at least 30 minutes.
If the wing and braces are still not flush, you can run the outward facing edge along the tablesaw and even it out. Sand off any burn marks and sand the rest of the wing.
Examine your handiwork!

Step 10: Add Some Flair to the Wing

We could easily allow the wing to stay square, but where would the fun be in that!

Using a random roll of electrical tape, trace out a curve on the corner of the wing.
Cut the corners!
Sand out any imperfections from the cut.

Step 11: Stain and Wax the Wing

The desk itself is black, so we decided to use our tried-and-true method for staining wood black on the wing.

Using chinese calligraphy ink and a shop towel, stain the wood! Once the wood has finished drying, it's time to wax it.
Like with the shelf, the easiest method is using your hands to apply the wax. Even it out with your hand and allow it to cure for at least 15 minutes.
Once it has cured, scrape any extra bits and buff it! It may require 2-3 coats of wax for its shine to really come out.

Step 12: Install the Shelf

Place the shelf on the desk where you would like the shelf to sit. Use painter's tape to get a general idea of where the shelf needs to be mounted to.
Mark and center punch the spots that you would like screws to come up from.
Make pilot holes where the screws need to be. Then using another drill bit that allows the screw, but not screwhead, to pass through relatively easily, "chase" that pilot hole.
Using a countersink bit, expose enough just so that the screwhead can sit flush with the internal surface of the desk.
Do this for all six holes (3 per side) of your shelf. Remember, this desk is the kind that can be opened up. Put the screws into the desk from the bottom side so that it comes up through the top.
Place the shelf on top of the screws that are sticking out, adjust as necessary, and then hammer gently (with some protection between the hammer and shelf), so that the screws mark some center punch points for where you need to create pilot holes.
Drill the holes for the screws. This time you want to pick a bit that allows the wood screws to bite into the wood a bit.
Screw the wood screws in and affix the shelf to the desk!

Step 13: Install the Wing

Using the same set of steps as with installing the shelf...

First, tape showing where the borders of the shelf are. Find the mounting points you would like to have screws in along the braces.
Drill pilot holes for the screws, then drill using a bit that allows the screw through easily. If possible, use the countersink bit to allow the screwheads to be countersunk.
Next, use a drill bit that allows the screw to bite into the wood and drill pilot holes into the mounting end of the wing.
Remember the screws will go from the inside of the desk to out!
Screw the screws into the wing braces!

Voila, the wing is set!

Step 14: Polish Up the Drawer and Install As the Finishing Touch!

Our particular sewing desk also had a drawer that came with it. It needed a little bit of additional love!

After taking it out and making sure all of its joints were properly glued and nailed together, we applied some wax to it! You can really tell the difference between where we had applied some and what its condition was previously.
Using the same tricks we've used as before when applying wax, remember to allow the wax to cure before buffing.
Once that has been finished, reinstall the drawer.

Step 15: Enjoy!

And after all of that work, you now have your own customized seamstress/computer desk! We will be using it as a computer desk, but you can use it as any other kind of desk you need!