This tutorial has perhaps been a long time coming as it is one of the more difficult things to get perfect when attaching a straight sleeve in to any of our patterns. When sewing the ZWGatherDress, ZWCroppedShirt or ZWWorkwearJacket your sleeve underarm needs to be sewn in a way which leaves the bodice side seam out of the way, so it doesn't catch in and hang weirdly here, and also so that you don't have a small hole at the underarm point.
Below is a simple step by step tutorial showing this process. Sewn with a mini size sample, cross hatched side shows wrong side of the fabric...
Start by pinning your sleeve into your armhole, matching the shoulder notch and underarm seams, right sides together.
Make sure you have snipped into the underarm notch 1cm / 1/2 inch so that you can keep the bodice side seam out of the way. The best way to describe this is to to keep your side seam at a right angle to your armhole when you are sewing across this section.
Stitch and overlock your sleeve into the armhole making sure you sew right into this underarm point so that you don't end up with a small hole here.
Now you should have a nice underarm finish here where the sleeve moves freely and the side seam sits nicely, pressed towards the back.
I hope this is helpful!