Do you mean putting a little flare around the wheel opening, if so I would like to know the same thing. Like this?
For little ones like in the picture, bend up 3/8" steel thinwall hydraulic tubing by hand to match the contour of the fender lip, weld it all the way around, continuous on the side towards the wheelwell opening, and then fair it in with lead all around. For anything larger in diameter, bend up 3/4" or 1" thinwall tubing with an electricians hickey, weld it all around as previously, then use many 3" square peices of #20 gauge steel to fair it into the original body panels, then use lead to smooth it all in. If you use bondo and not lead, make the first one or two coats from the green fiberglass impregnated bondo to waterproof any pinholes, then use the lighter air entrained bondo for final finishing. I did a thousand of 'em back in the seventies.
I believe the ones pictured are the fender lip off of a 55 chevy or a tri 5 if I am remembering correctly.