Set two 12-foot-long, 2-by-4-inch hardwood boards next to each other length-wise on your workspace floor. These boards make up the outside rails of your ladder.
Draw a line across the rails width-wise 8 inches in from one end. Then, draw another line every 15 inches. The last line will be 8 inches from the end of the rails.
Connect the rails with nine 19-by-2-by-3-inch hardwood boards. These boards are the treads of your ladder. Place one tread on each set of lines. Use a cordless drill to drive two 2 1/2-inch deck screws through the rail into both ends of each tread. The screws should be 1 inch apart.
Check that each tread is perfectly square, or at a 90-degree angle from the rails.
Screw an L-bracket centered beneath both sides of each tread, securing the treads to the rails. Use 1/2-inch wood screws and the cordless drill to attach the L-bracket.
Fit 4-inch-long, rubber ladder feet over the bottom of the rails to prevent the ladder from slipping.