Okay here's a picture:
I put screws about every 6-8 inches.
Start in the middle of the fender and work out. You can play around with how far in you push the fender to make it have a better profile. I found it best to have the flare sit in a little at the top of the fender, and I cut the lip so they could stick out more at the bottom.
like yea:
You can see in these pictures of my truck that the top is a little narrower than the bottom, but I should have done more
If you want better pictures I can
Maybe I should do a write-up for the tech library on Home Depot fender flares...