Front or back, it’s the same process. Line up and mark where the mudflap needs to be. Then drill holes in both items – mudflap (if not pre-drilled) and mudguard. If using pop rivets, be sure not to drill holes which are bigger than the rivet diameter, and use washers both sides.
A better way might be to use nuts and bolts as they are easier to remove provided you use stainless items.