What JDW said. It is also wise to site down the edge of the 2x4 to see if it is cupped and put the convex, belly side down toward the plywood. If you can, two finish nails will hold the two pieces of plywood from sliding on each other while you are setting up the clamps

You can also weight down the sandwich with something like cinder blocks if you don't have clamps although the clamps are a surer way to get good contact.

So many guns, so little time!