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.