The flap mounting wasn't too terribly difficult to do. I first used a piece of cardboard to make a template, then cut the pattern out of a piece of 1/8" plate (a little on the heavy side, but what I had laying around). Cutting metal this thick with a saber saw is probably the most hassle, but if you let the saw take it's time, it's not so bad. Then some BF hammer time to shape the metal to the arc of the fender well.

One thing I did do that would require a welder is, I did weld a piece of pipe on the back side where the bolt circled in yellow goes through and with the pipe (as a sleeve) is bolted to one of the beefy running board mounting holes. This is the strongest connection by far. The bolt in red goes through a pinch seam hole that you can see (and find by opening the door and looking down between the fender and the body).

Screws in blue go into fender sheetmetal holes and would be pretty weak, but strong enough to clamp the flap between the fender and metal plate.

Screw holes in green go thru the plate to a companion plate to clamp the big flap on. In this picture, only the smaller flap is on as I was in the middle of cutting a new big flap to drop it down a little more to keep the rocks from hitting back near the back door. It needs to be down to 5" off the ground on a 4Runner with 31" tires and no lift.

Where you see the red line is a bend in the metal that is critical for tire clearance, as it is near the big bolt in back where the tire comes closest. I ran the 4Runner up a bank to compress the spring as far as the IFS would go, and turned the steering lock to lock to check this out. If you need more clearance, (on the 3rd gen) there's a body pinch seam directly behind the red line that you could hammer over flat and it would let the plate go back closer to the body and you probably won't need the bend at this line.

Does this picture remind anybody of an Arlo Guthrie song from a few years back?