ive had to do it on my car and a couple of 5.0's. It seemed to work best with some tight fitting tires and a basball bat. Just insert it the crease and roll the car forward. then do the same thing backwards. it takes a long time to get it to start creasing right .lots of back and forth's
I've done the bat method before. IMO it works best if you have a couple guys stand in the trunk. Start with one or two and add until you get the lip rolled right. You really need the extra weight to get the right amount of force. Most recently, I had a body shop trim mine out a bit and then hammer them upward a little. That worked well too.