You can hem your own pants (yoga or otherwise) in just a few easy steps, even if you don't own a sewing machine. If your yoga pants are fairly close to the right length, this method, based on instructions by blogger Dacia Ray, is fast and easy.
Instructions
Leave the original hems alone for now, and decide how much shorter you want the pants to be. Take that number (in inches or centimeters) and divide it in half. Example: If you want to take off 2 inches, use 1 inch as your measurement.
Fold the pants up so they are cuffed with the insides facing out. (Do not turn your pants inside out.) Measuring 1 inch, or whatever length you've chosen, from the original hem down to the folded edge, pin all the way around the leg. Be sure to keep measuring all the way around for an even hem, and don't include the original edge of the jeans in your measurements.
Once you've entirely pinned both hem, it's time to sew them up. To imitate a sturdy machine-sewn hem, use a backstitch all the way around, sewing alongside and above the original hem. Use the original hem as a guide to create a nice even stitch.
When you're done sewing, trim the excess material leaving 1/2 inch to prevent fraying, and press it down with an iron.
Turn the leg right side out again, and you'll see the pants' original hem. Press the leg flat with an iron, and you're done.