plywood joinery typically needs reinforcement
make doweling jig from wood or aluminum
Use a Doweling Jig to Drill the Face Holes
Plywood joinery also typically needs reinforcement. A butt joint -- one piece of wood butted up to the other (Image 1) -- is a weak joint, but it can be strengthened by adding dowels.
The first step is to drill properly sized, properly placed holes for the dowels. The holes on both sides of the joint must line up properly. You can measure each hole, a tedious and undependable process, or buy one of many available doweling jigs, or you can make your own jig from wood or aluminum (Image 2).
Place the jig where you want one work piece to line up with the other and drill the holes.