A good source for making leather recoil pads is "Shotgun Technicana" by McIntosh/Trevalion-Chapter 11. MidwayUSA also has a free video which shows how to made leather recoil pads.

The pig skin or goat skin should be approx. 0.025 thick. Be very careful when you undercut the rubber recoil pad. The undercutting is the thickness of the leather used. When finished, the transition from leather to the stock should be smooth.