Determine the width of wooden brace you need by measuring the width of the wooden shelf. The width of the brace will be 1 inch shorter than the width of the shelf it will support. As an example, a six inch wide shelf will require a five inch wide wooden brace.
Draw the figure of the capital letter L on a piece of paper by tracing the edges of a square measuring tool, but extend the horizontal and vertical lines in the figure so that the length of each line will be equal to the width of the wooden brace. Draw a straight line from the top of the vertical line to the right end of the horizontal line to form the figure into a right triangle. Using the example in step 1, the triangle would end up with two sides measuring 5 inches each, and one side measuring 7 1/8 inches.
Cut out the figure with scissors and place the triangular piece of paper on top of a piece of plywood between three-fourths and 1-inch thick.Trace the outline of the triangle with a pencil and cut the outline with a wood saw.
Stand the triangular piece of plywood on a table in front of you with its longest side resting on the table and its 90-degree corner pointing up. Label the 90-degree corner as B, the right corner as C and the left corner as A.
Remove the pointed ends of the triangular plywood by cutting away one-half inch from the tip of corners A and C, using a wood saw. Following our example, measure one-half inch from the tip of corner A, then draw a line perpendicular to the 5-inch side of the triangle. Saw along the line to remove the pointed tip of the plywood. Use the same procedures to remove the pointed end of corner C.
Measure the length of the wood from corner A to corner B. Cut a rectangular piece of plywood 3 inches wide with a length equal to the measurement you took. Following our example in Step 5, you will have a rectangle measuring 3 inches wide by 4 1/2 inches long.
Lay the rectangular plywood wood on a flat surface, then mount the triangular plywood upright on top of the rectangular plywood. Position the triangular plywood so that both pieces are parallel to each other and the short side of the triangle is resting along the center of the rectangular plywood. Following our previous example, the 4 1/2-inch edge of the triangular plywood will be seated lengthwise on top of the rectangular plywood.
Align both ends of each wood flush then nail both pieces together. File the edges of the wood to make them rounded, using a wood file. Smooth all parts of the wood with sandpaper, then apply paint or plastic varnish as finishing.