I make and sell pillowcase dresses for girls have made hundreds. I figured the armholes out by trial and error and searching the net for the best way. I hope I can explain in a way you understand, I'm not too good at explaining.

I cut a bias strip longer than the armhole. I use my 12" ruler to mark them off. I trace down both sides as a guide. This will give you a strip a little longer and a little wider than you need. I use the assembly line method so it is just easier for me to cut an all purpose size and cut to fit later.

You must cut them on the bias in order for the fabric to fit the curve of the armhole and lie smooth when finished.

Sew the strip along the edge of the armhole right sides together.

Trim the seam allowance to approx 1/8 ". If you do this you won't need to clip the curves of the armhole.

Press the armhole so that the seam on the wrong side is pressed down toward the dress. It should be lying flat open on the right side at this point.

Trim approx 1/4" off the outside edge of the bias strip.

Fold the strip in half toward the inside of the dress, and again once more. At this point the strip should form a perfect facing with the finished seam forming the armhole. Continue to work your way all the way around the sleeve pinning as you go.

When both armholes are pinned, sew it on the machine with the outside of the armhole on the bottom, and the inside on top.

Press smooth after you sew.

Hope I was clear enough and this helps.

It would be nice if the op would come back and say something,like thanks or that was of help to me etc.JMO
Kathi