How many short rows you'll do and where you'll start will depend on your guage (stitch and row) and how much length you want to add. But if you start wrapping and turning closer to the middle and make your short rows longer and longer (picking up the wraps as you go past them), your darts will go in the right direction. If you need help with the math, let me know.
Trying to think of an example....
Say your sweater was 200 stitches around, 100 on each side, and your stitch guage was 5 and your row guage was 7. To add one inch, you'd do 3 wrap and turns on each side. Remember, every wrap and turn gives you 2 new rows! Do your first wrap 5 stitches from the left underm, w&t, purl back to 5 stitches from the right underarm, w&t, knit to 3 stitches from the left underarm (picking up your first wrap when you pass it), purl to 3 stitches from the right underarm (picking up wrap), w&t, knit to 1 stitch before the underarm, w&t, purl to one stitch before the underarm, w&t, and continue knitting around, picking up the last wrap on the next round.
If you wanted two inches, you'd start 10 stitches from the underarm, given the assumed guage.