The top pair are for the rear, to install the rear; remove the OEM pop rivet and the bolt on the bottom of car, put the mudflap on car making sure that the "hook" is attached to the car and replace the pop rivets with the ones that came with package then screw the bolt back in. Boom rear is done. SAVE THE OEM POP RIVET! For the front I would suggest buying some u-nuts from an autopart store, take the mudflap with you. The fixing screw goes through the top hole use the u-nut with the screw to attach the mudflap, the bottom two are for the pop rivets.
They sent extra pop rivets and screws so its ok if you have extras.
So the OEM rivets you replaced, look in the rear fender well. Do you see three holes that look like pop rivets fit inside them? Use the OEM rivets you replaced to fill those holes. I hope all this makes sense.
PM me and maybe we can facetime/video call when you install if you need more help.