Line up one of the long boards flush along a long side of the the plywood, with the plywood sitting on the broad side of the board. Place the other boards beneath the plywood to help it lie flat.
Nail the plywood to the first board roughly 2 inches from each end of the plywood. Put two more nails equally spaced between the two corner nails.
Repeat step 2 on the other long side of the plywood with the other longer board. Each of the plywood's long sides should now be supported by a two-by-four.
Cut the shorter boards to fit snugly between the longer ones.
Position two of the shorter boards flush beneath and along either end of short sides of the plywood.
Nail the shorter boards to the plywood the same way you nailed the longer ones. Also nail one shorter piece in the center below the plywood.
Lean your rebound wall against a wall or tree, trying different angles to change the rebound trajectory of your shots.