My understanding is welting is optional. I'm not even sure what purpose it serves except maybe as a "gasket" to fill in any spaces the flares don't conform to, but when I test fit them without the welting before the final install, I didn't find any open space.
When I did mine, I took a pencil on a jig set to halfway across the flat part of the flare, which I measured as 1" wide. So the edge of the jig followed the top edge of the flare and the pencil drew a line 1/2" down all the way around. I started with the Driver's rear because I had to account for the gas caps. Then I did a best guess for the "center" of the flat top section and lined that up exactly over the axle and marked it on the line that's 1/2" down. I then ran a string along the 1/2" pencil line so I could get the total distance, a little math, and it turned out a screw every 3" in both directions made a nice, even distribution with a screw ending up at the bottom of the front and back for a total of 23 screws. There's a 24th screw on the rear wrap around facing the ground. This method also put a screw just above the rear lower body line so it holds it tight around that edge.
The fronts were done basically the same - scribe a pencil line 1/2" all the way around. But the fronts are shorter so I played a little with a starting point, basically the wrap around towards the front facing down, then measured every 3" all the way around. It worked out very good, ending at the bottom of the rear, with a total of 22 screws, although the top screw isn't exactly over the axle. If I put it there, the ends wouldn't line up right. But I did mark the "center" of the flat part of the front flare and lined that up with the axle when I was ready to drill. I wanted it to "look" right, like it belonged there. I've seen sloppy work where it's shifted front or back and it just looks bad compared to the tire.
I drilled pilot holes in the flares at the marks while they were off, then held them on using "C" clamp vice grips. I adjusted and adjusted until it looked good, then drilled through the pilot holes into the metal. Make SURE the top of the flare is parallel to the body line. It makes all the difference.
Once I had all the holes drilled, I pulled the flares off and used 1" blue painter's tape, taping along just below the holes all the way around. The bottom of the tape was now my cut line for the fender, 1" below the holes, but you can actually cut it lower - 1.5". My Bronco wasn't painted at the time I did this, but I still ran blue tape along the upper part of the path to prevent my jig saw base from scratching the metal. I used a new, metal cutting blade and went slow.