I tend to wear holes in the knees of my favorite jeans, which makes them look kinda shabby. Cutoffs are a great way to keep a favorite pair of pants, even if they're worn out.
Take your old pair of jeans and lay them out.
Place a pair of shorts that fit you well over the jeans. Align them at the crotch and pin them down so they stay together.
Then, mark on the jeans along or below the edge of the shorts. Make sure it's even on both sides and that there's a slight "V" shape to the lines, which makes it more flattering.
Cut along the lines you made
Optional: Fold up the hems and iron them down. You could also fold the edges in and sew a hem, or just let the jeans fray.