Measure and note the width of the toilet tank. Add 3 inches to that dimension to calculate the length of the shelves. The top shelf will be 4 inches wider than the other shelves. Measure and note the height of the top of the toilet tank.
Mark the mid-points of the widths of the ends of two 1-by-10-by-72 boards. Make marks 3 inches to either side of the mid-points. Draw a line lengthwise through the center mark on each board, perpendicular to the ends. Mark the lines 3 inches from the ends of the boards. Draw to connect the marks to form a V at the end of each board. Use a jig saw to cut out the V-shaped notches.
Lay the trimmed boards on a flat surface, side-to-side, with the notches facing the same direction. Align the ends of the board. Measure from the bottom notched end of a board to mark it where the lowest shelf will be installed. The lowest shelf should be about 2 inches above the top of the toilet tank. Use a carpenter's square to scribe a line a across both boards. Decide where you will position the two shelves between the top and bottom shelves. Draw a line across both aligned boards at these positions.
Cut a 1-by-3-by-48 board into six 8-inch lengths. These narrow boards will become support rails for the ends of the shelves.
Apply glue to one side of each support rail. Center the support rails on the notched boards so the upper edge of each support rail is aligned to the lower edge of the shelf-positioning lines. Place a weight on each rail to prevent the rail from shifting as the glue dries. Allow the glue to thoroughly dry according to the manufacturer's directions. Remove the weights.
Draw a line down the center of the support rails from end to end. Use a countersink drill bit to drill evenly spaced pilot holes through the lines, about 2 inches apart. Attach the support rails to the boards with drywall screws and a drill and driving bit. Fill the countersink holes with wood putty.
Assemble the shelving unit on a flat surface by resting the vertical supports on their long edges with the shelf-support rails facing each other. Insert the shelves on their support rails. Rest the top shelf on the tops of the vertical support boards.
Beginning at the top shelf, use a carpenter's square to square each joint before you screw the shelves and supports together. Center an L-shaped corner bracket over each of the two joints between the vertical supports and the top shelf. Mark the screw-hole positions, then remove the brackets. Drill pilot holes with an 1/8-inch bit through each mark. Reposition the brackets, then screw them to the shelf and vertical supports with drywall screws. Repeat to attach remaining shelves, except the lowest shelf, to the vertical supports.
Draw two lines across the top shelf — from front to back — aligned with the centers of the tops of the vertical supports. Use a countersink bit to drill evenly spaced holes, about 2 inches apart, through the top shelf. Secure the top shelf to the vertical supports with drywall screws. Fill the countersink holes with wood putty.
Remove the lowest shelf. Stand the shelving unit upright on the floor. Use wire nails to nail the ends of the shelves to their support rails. Each joint should be secured with three or four evenly spaced nails.