After a hide is tanned, it stiffens after drying. The process to make them soft is known as "breaking the skin." To soften the hide, the process requires a breaking tool, as chemicals and oils may preserve hides, but don't soften them. Professional tanneries typically place tanned hides into big tumblers with sawdust and use gravity and tumbling to make the hide soft and pliable. If you plan to soften several tanned hides, you can make your own breaking tool at home.
Cut the Radius on the Board
Place the paper plate on the end of the board to use as a template and trace its edge to create the line for cutting. Using a band saw or circular saw, cut on the line to form a half circle on one end of the board.
Make a Smooth Rounded Edge
Use a router on either side of the board with the convex bit to create a 1/8 inch edge on the end of the board. Change the bit to the flat-edge cutting bit and continue to further recess the edges of the initial routed edges. Raise the router guide each pass to remove material that might create drag on the hide as it passes over the breaking tool, while still leaving an edge strong enough over which you pass the hide. Make certain the edge is smooth and there are no splinters or bits of wood that make break away. Sand the edges to remove burrs.
Drill Holes and Attach Board to Worktable
Drill two holes equally spaced apart about 1 to 2 inches in from each side on the bottom of the board. Drill two holes in the edge of the worktable. Install the carriage bolts and tighten to securely attach the breaking tool to your worktable.
Soften the Hide
Grab each edge of the hide and pass it over the routed edge of the breaking tool board in the same way shoe shiners use a rag to shine shoes, pulling the hide back and forth across the board to soften it. Repeat this for all areas of the hide until it is soft.
TL;DR (Too Long; Didn't Read)
If you don't have a worktable, and plan to tan and soften multiple hides, insert a 4-by-4 inch post into the ground at the appropriate height -- keeping in mind the height of the 16-inch board -- and add cement to secure the post firmly. Attach the breaking tool to the post.
If you just need to soften one tanned hide, simply find a good solid edge that does not contain any splinters, burrs or objects that can rip the hide, like on the sanded edges of a saw horse. Grab two sides of the hide and rub or buff it across the edge as you pull it taut until it softens. Repeat for the entire hide. This process breaks the hardness of the hide. After softening, apply an oil specifically for keeping hides soft.
Do not use soft woods like pine to make the breaking tool, which might break when buffing the hide across the top of the board. Oak also does not work well since it splinters and is too brittle.
The breaking tool requires a firm attachment so that it does not move when you buff the hide against it.
Always wear the appropriate safety gear when working with construction tools such as safety glasses, ear protectors and gloves to avoid getting splinters.