Step 1: What You Need
An old pair of jeans(or just a pair of flair or straight leg jeans)
Step 2: Getting Rid of the Top Stitching
turn your jeans inside-out and then sew them so that you cant see the seam.
Reinforce the crotch seam.
Step 3: How Skinny Do YOU Want Them?
Then measure how many inches you want to take off of each side and mark it with a sharpie or marking tool. Mark it again where you want it to start getting skinnier(somewhere around the knee)
Step 4: Finnish Marking
Make sure you take off the same amount on both sides.
Step 5: The Fun Part
Don't cut off the extras unless your sure that's the way you want your skinny jeans.