Both. For the front splash guards, you need drill a 1/4" hole inside the wheel well. On the rear splash guards, you need to drill two holes. Very easy mod. I highly recommend that rivet tool.
Speaking of which, I bought the mud guards and for the heck of it, ceramic coated them. Have to do the inside part tonight. Saw the trick on a website. I wanted to see if it ruined the appearance. It looks good, nice and dark, not too glossy. I think I will put a few coats on them and probably coat all the plastic trim on the Jeep once it comes in, sick of having the plastic look like crap after a few years. I used some stuff called amour shield, a couple detailing websites recommended it. I also coated some 15 yr old trim on my PT, surprised me, blackened it up just like new and gave a semi gloss appearance.