Wash and dry the jeans first
Put the jeans on and mark where you want the rips
Lay the jeans flat on a hard surface
Place a piece of cardboard inside the leg behind the marked area
Use sandpaper, a pumice stone, or a razor to rough up the fabric
Make small horizontal cuts with scissors or a craft knife
Pull out a few white threads with tweezers if desired
Rub the cut area to fray the edges
Repeat on other spots as needed
Wash the jeans again to soften and finish the rips