Sometimes the pants are boring. Sometimes they're stained. Sometimes they are velvet and have big worn spots... In any case, they need a little something to give them a new lease on life. Here's a technique to rejuvenate a favorite but ailing pair.
You'll need a sewing machine and some embroidery floss in a color that complements your pants, and regular sewing thread (in whatever color you want, this will show somewhat). You can also use appliques (pre-made or by using stitch witchery). I used some pre-made iron-on flowers I happened to have.
In this Instructable I will assume familiarity with sewing, and with your own sewing machine. If something is confusing, please comment and I'll do my best to clarify.
Step 1: Open the pants seam
Rip out the inseam of the pants, from cuff to cuff. You'll need to undo the hem for a little way on either side, or if the pants are too long, you can cut it off and rehem at the end. This is so you can open each leg out and stitch on it without having to shove the sewing machine down the pant leg.
Many commercial pants are stitched with two seams: a chain stitch to hold the pieces together, and an overlock stitch to keep the edges from fraying. The overlock has to be cut pretty much all along its length, but a chain stitch, if you mess with it a little, will come completely unraveled from one end to the other. It's hard to tell which end to pull, though, so try each leg.
Open out your pants and draw a pattern on the inside, remembering that it will be mirror-image on the outside of the leg.