Turn your item inside out and pin the raw edges
Your secret weapon on this step is pins. Lots and lots
of pins! Especially with light, stretchy or knit
garments, pins will help keep all those edges lined up
they way they should be.
Take extra care that you line up the two pieces at the
sleeve end, under arm, and bottom of the garment. These
are the places you will most notice a misalignment.