Thankfully, since distressed jeans are fashionable, there are TONS of guides out there on how to do it! Here’s a few methods you may find helpful:
You can also use heavier grits of sandpaper (lower numbers) and elbow grease to not just soften, but to actually wear holes through the jeans.
Make sure you concentrate the distressing on areas that jeans would naturally get distressed – the fronts of the thighs, the knees, around areas that are on the tops of folds and creases (rather than inside the folds and creases) when worn, around the pocket openings and the bottom hems, etc.
Have fun with it! I hope that helps :]