There are several websites that have posted instructions. Here's one: http://www.customcalls.com/makeaturkeyboxcall.htm
I looked over several before starting this. My hunting pal (who already has a slate call) is a gadget guy, and teases me for my pennypinching, so I decided to use all salvaged materials. This call is mostly poplar from an old dresser, with a handle made from a paint-stirring stick (just for the logo, really).
I like the sound; if the turkeys agree, I'll probably make a few more.