This is easy to show but hard to say. You really don't want to match the cut edges of sleeves/armhole. What you want to match is 5/8 in from the edge. Since one curve is an outside curve and one side is an inside curve the cut edges will be a different length. Now having said that. you need to feel with your thumbs/fingers at the stitching line before you sew for any places that will pucker. Run your fingers one on top and one on back as if your fingers were sewing, squeezing them together a bit. If you push one fabric more than the other then you will get a pucker when you sew. sometimes it is just a matter of distribution .
Be careful to sew at the seam allowance not inside or outside. If you run gathering stitches on the sleeve, draw them up to the size you need, smooth out the gathers, or even them up. and using the tip of your iron, iron only the seam allowance. You can smooth out alot of fullness this way. Sometimes it doesn't look like there was any gathering if I iron the sleeve first. Caution here, if you used the washout marking pen to mark the seamlines like someone else suggested, do not iron. Some can be set with an iron. Actually marking the sewing line is not a bad idea, just don't do both, marking and ironing.
Now when I pull the gathers, and any time I pull gathers, I pull them more than I need and then let the fullness back out. What this does is help distribute the fullness into any fiber that can squish, and not just some of them. I have found that I get much smoother gathers if I gather the threads all the way and not just partially, then let them back out to the length I need. .
Also if you can get ahold o fone of the video by Margaret Islander, she shows you how to put in a sleeve with the sleeve on the bottom with no gather stitches, like they do in factories. This is the absolute best way for casual clothes, but if you are tailoring a jacket, and for some sleeve styles, they have to be sewn in, in the round.
Sandy E