Measure your armhole and cut a strip of bias tape of the same length + 1 cm for seam allowance. Stitch the ends with right sides together.
Then turn toward the wrong side of the dress and pin.
Don't skip this step! I did, and as you can see from these photos, the edge looks quite messy and it doesn't lie flat. Never underestimate the power of your pressing iron!
If you want to see another alteration, check out Eléonore's post!