This is an ultra-easy project that doesn't even require a sewing machine. All you need is a suitable pair of stretch capri pants that fit you and are a little loose at the hem, a pair of shank buttons, some ribbon, and basic hand-sewing supplies and skills.
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
The 18th century happens to be my favorite costume period. These faux breeches work because they mimic a couple of key features of the real thing: breeches were relatively fitted (depending on the decade) and they often closed just below the knee with a cuff that buttoned. By rolling up the hems of the capris, we get the same look. They should be loose enough that you can do a little foldover (towards the back) and pin the fold in place. Make sure the cuff is nice and even all around, and that both cuffs look the same. Use multiple pins if necessary.
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
Sew the buttons on top of the folded-over part of the cuffs so that it looks like they are buttoned closed. If you need more specific directions for this part, see below.
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.
Step 3: Add ribbon
Tie your ribbons around the shank of each button.