1. Cut the ends of the flare so that it sits flat against the body. Leave a small triangular piece of material so you can drill through it.
2. Line the flare up more or less where you want it and drill through both the flare( piece of material you left) and the inside of the fender.
3. Place a screw in that hole and secure that corner to the fender.
4. Pull/ stretch the flare by hand over to the other side. It's very flexible and won't break. Repeat step 2 on this corner.
At this point you will have each corner of the flare attached to the fender by a screw on each corner. As it is, it will naturally sit flat because of the tension of the stretch.
5. I then went about 3/4 of the way up and drilled a hole through the inside lip of the flare as well as the inside edge of the fender and secured with a zip tie. Repeat on the other side.
I don't have anything at the very top of the flare, it's really not necessary. Although, I have been playing with the idea of magnets, but I digress.