For a truly authentic vintage knitwear look, you just cannot beat a vintage pattern. And there are hundreds of these patterns available online for free! A quick search in the Ravelry database, or even Google will yield an amazing number of gorgeous possibilities.
[The pattern for this particular sweater may be found here.]
Step 1: Terminology
One thing to keep in mind is terminology. For instance, substitute "jumper" for "sweater" in a google search, and British vintage patterns will jump to the top of the list.
Other words that will help to narrow a search are cardigan, crew-neck, pullover, lace, bolero, and shrug.
Step 2: Needle Sizes
Something to be aware of is British vs. American needle sizes. When in doubt, check the mm measurements and make sure that the gauge and yarn size makes sense.
My favorite decades are the 1940s and 50s, and vintage sweaters from these eras generally utilize very thin yarns and needles.
American needles use small numbers to describe small/thin needles. British needles are sized in the opposite direction. So, a size 0 needle in a British pattern would require bulky yarns and a size 0 needle in an American published pattern is very thin and delicate.
Step 3: Sizing
Quite a few vintage knitting patterns come in one size. With the exception of intricate lace pattern repeats, these are generally easy to size up by calculating gauge and adding or subtracting to or from the initial cast on.
Another option is choosing a thinner or thicker yarn and a suitable needle size to make the existing pattern smaller or larger as needed.
Just make sure to calculate how those changes will affect the finished size of the garment!
Step 4: Charting
Most of these vintage patterns will not have a chart.
I find these incredibly helpful, so I make my own. Changing the prose instructions into a form that looks something like the actual knitting pattern helps me learn a new lace pattern so much more quickly!
Standard chart symbols include a dot to indicate a purl stitch and a circle to indicate a YO (yarn over). If there is a stitch I am not sure how to chart, I will pull out a contemporary pattern for hints.
HINT: Remember that these charts are read right to left for the odd numbered rows, and left to right on even rows.
Step 5: Buttons!
And for a lovely vintage finish, try some Dorset Buttons! There is a fabulous tutorial over at BurdaStyle. It is the perfect way to make some matching buttons (something I always have trouble with finding!).
I like to use plastic rings from the home decorating section of my local fabric store for a solid base to start - this makes it easy to slide the button through the buttonhole.
Another vintage option is knitting a small square and stuffing it with yarn scraps before sewing it to the sweater in place of an actual button.