Here's some pic's of the models I have done using things like the Jello cups.
pick a product that fits your build ( look wise ). You will take the cup and use the bottom side of it as the ( top of the fender ) the roundness is what gives you the flair look and style for you build. You then line it up to your stock fenders trace the wheel well out on to your Cup and then cup it out. Mark out what you will need to cut away from your stock fenders then glue into place ad filler / putty to shape into your to your build then sand when ready. The rear can be done the same way but with less flair.
The Bonco was done this way. the back fenders are just the newer year molded in.
The Coyne TT was done using this also , both front and rear.
The Ranger the same.
Chase 1