I made these for a pirate-themed party to go with a (much more elaborate) period justaucorps.
Step 1: Try on the capris and cuff the bottoms
I made the cuff with two folds; more than that would make them so thick as to be difficult to sew through, and too bulky.
Step 2: Sew on buttons
I also discreetly sewed the cuffs in place on the other side of the leg, opposite the button, so they don't gape.
Sewing on the button:
Use doubled thread that is knotted together. You are sewing through quite a bit of fabric, so be generous with your thread and go over the spot with a few stitches before you add the button. Sew through the button and all layers of fabric five or six times, then secure the ends on the inside with several stitches in place and then a big fat knot.