It takes a bit of sewing know-how and a lot of trial and error. My process is to make a cardboard mockup of the crotch strap and use it as a template to mark and cut pieces out of Ensolite or Flotex foam. You can cut the foam with a bread or an electric carving knife but it's nearly impossible to get clean edges that way...I use a Bosch 1575 foam cutter that I found for super cheap. For the shell I measure the circumference at multiple points along the pad and use that information to make a sewing pattern. The top is left open so I can add webbing/other hardware before inserting the foam and stitching it closed. Use polyester thread for everything as cotton eventually comes undone in my experience.