The only advice I can give for sewing that armhole is just go slowly. A little puckering is fine. Remember, it will be hidden by an arm

Also, I try to pull the armhold straight instead of sewing it like a curve. It tends to sew more smoothly this way for me. And, I flatten as I go. Leave your needle in the down position that way you can adjust without losing your spot.
I usually use bias tape because I hate sewing those little folded hems on those armholes. This is a great tutorial on how to sew bias tape:
http://angrychicken.typepad.com/angry_chicken/2008/03/bias-tape-tutor.htmlAmy Karol shows us how to sew it on perfectly!
I hope this helps you out some.