Step 1: Design your board
In this files section are two CAD files (DXF and Corel Draw) of my favorite board.
I printed my design out on a large format printer. You can also just draw your design my hand on a large sheet of paper.
Step 2: Cut board
Rough cut the shape with a table saw or jig saw, and then smooth the edges with a belt sander. Round any sharp edges so the board doesn't cut you, but also so it's not prone to knicks and dents that will allow water through the epoxy.
Make sure you align the grain of the wood on the face of the plywood along the direction of the board.
Drill holes and install 1/4-20 tee-nuts for mounting footstraps and a leash attachment point.