All right. You've knit the body up to the arm pits, right? Set it aside. That's right, take a deep breath, set it aside as is. Hug it first if you want to. Get yourself some DPNs or a shorter circular needle, or whatever you need to knit cuffs, and get ready for the next.
Let's take a look at our handy schematic again, hmmm?
If you're knitting a generic pullover, make the cuff match the ribbing you already knit at the waist; use whatever needle you used before, and whatever ribbing. If you knit a hem for the waist of your sweater and have no other plan, ribbing is a good way to go for the sleeve cuff. Use a needle one or two sizes smaller than the main body is being knit on (makes the ribbing stretchier) and knit a couple inches' worth. Consider a tubular cast-on and a 1x1 rib.
20%. You can get it with a calculator, pressing "[100% figure] x 20% =" I used 222 stitches (remember, steek stitches do NOT count in your 100%) for the body. So, for the cuff of my sweater, 222 x 20% = 44 stitches. I'd cast on 44 stitches.
Once the ribbing is knit, switch to stockinette stitch, the needles you used for the body, and knit three rounds plain. On the fourth round, mark a column of stitches to function as the underarm stitch. Increase one stitch on each side of the underarm stitch. I use the make one (backward loop) increase method.
Increase two stitches, one on either side of the underarm stitch, every fourth round, until you reach 40%. "[100% figure] x 40% =" or in my case, 222 x 40% = 88 stitches. After that, knit until the sleeve is as long as you want. Remember, you're knitting to your arm pit, not your shoulder.
Then, knit another one.
To avoid the cliche of sleeves two different lengths, the best and easiest way is to make sure you've got the same number of rounds/rows in each sleeve. I do it by tying string into the first sleeve, every twenty rounds. Then I knit the second.
Next up? Coloring outside the lines, on the sleeve cuffs.