Place two 6-foot long, 10-inch by 1-inch boards on the ground, parallel to each other and set upon their edges.
Position three 4-foot long, 10-inch by 1-inch boards at right angles in between the 6-foot boards. Space the 4-foot boards two feet apart.
Screw the boards together, from the outside of the 6-foot boards, using 2-inch wood screws, spaced three inches apart. Use an electric screwdriver.
Place a 4-foot by 4-foot sheet of 1/2-inch plywood onto the structure. Carefully place the plywood so that it lines up with the edges of the 6-foot boards and is evenly spaced between the top and bottom edges of the structure. Screw the board onto the shelves with wood screws.
Set the shelves upright and place them against the basement wall, with the plywood side against the wall. Use concrete anchors to permanently attach the shelves, if desired.