Before you do the initial sewing, pin the muslin together and have the wearer (whether that's you or some other person) try it on first - make sure shoulders and such fit - and make adjustments accordingly. It's MUCH easier to do a fitting when things are pinned, because if they're sewn, you have to rip and re-stitch, and not all fabrics tolerate multiple rounds of seam ripping.

My mother used to make most of my fun clothes growing up, so I'm aware that this is A Thing that needs to BE done, but I'm not aware of HOW to do it *properly*.

(I've made a few clothing items, but more "medieval styling" than modern, so a much looser fit, deliberately.)