Cut the plywood sheet in half along the diagonal, making two right triangles. Set one triangle aside.
Cut the 2-by-2-inch post into two 30-inch lengths.
Cut the 2-by-4-inch beam into two 30-inch lengths.
Sand all faces of the lumber.
Set the two beams in an L-shape along their 30-inch sides with the long edge of one beam abutting the inside face of the other. Nail them in place with four evenly spaced nails. Drive the nails through the face of one beam and into the edge of the other.
Put the end of the L-shape in the right angle of your plywood triangle, its outside edges flush with the edges of the triangle. Screw it into place with three wood screws. Place one screw in the corner and the other two each 3 inches down either side of the triangle.
Attach one post in each remaining corner of the triangle, held in place with two wood screws driven through the plywood and into the post. Note that each post will rest slightly back from the tip of the corner.
Apply painter's putty to the heads of your nails and screws, and to any pits or divots in the face of our lumber. Allow to dry.
Sand off any excess putty and any remaining rough spots of wood.
Paint or stain to your taste. Allow to dry.