Re: (added photo) I struggle sewing arm holes. Is there a trick to it?
I don't know why the pattern would ask you to double fold on an arm hole? In my experience it is nearly impossible to get those flat. As some pp have said, bias tape etc. is much better. for the dress you made, a nice facing would have rounded it off nicely.
It is a very cute dress and I think you managed to do better with the double fold and stitching that I would have done. I really would have pulled out the bias tape or made my own from the dress' cloth.