If you build your own dock, you know you have to allow some height for the water level to rise and fall according to the state of the tide or other environmental conditions. Sometimes, the difference in water levels may be great enough that the only way you can get into your boat with your dignity intact is with a dock ladder, allowing you to climb down properly, rather than making an unceremonious jump for your boat from the end of the pier.
Use your measuring tape to measure the distance between the height of your dock and the water at lower low tide. Add 3 feet to this measurement and mark this length onto the 1-by-4 inch boards. Use your circular saw to cut the 1-by-4 inch boards to this length, forming the side rails of the dock ladder.
Measure 3 feet from the top of each side rail and make a mark at that point. Measure downward from that mark on each side rail, making another mark every 12 inches to indicate where a tread (step) should be mounted. The top 3 feet of the ladder's side rails will not have any treads between the side rails and will extend above the level of the dock, so that a person climbing the ladder will be able to maintain a grip on the side rails and "step through" these handrails and onto the dock.
Lay the two side rails on edge, parallel to each other, with their tops even. Count the number of treads you will need. Use your circular saw to cut the 2-by-4 inch boards into this many pieces, each 24 inches long.
Place one 2-by-4 inch tread on its edge between the side rails, at the first mark. Drill two 1/8-inch pilot holes through one 1-by-4 inch board and into the end of the 2-by-4 inch board, then drive the 1/4-inch galvanized wood screws through the pilot holes in the 1-by-4 inch board and into the 2-by-4 inch board with your screwdriver. Repeat this process to attach the other 1-by-4 inch board to the end of the 2-by-4 inch board and form the first step.
Continue attaching steps between the 1-by-4 inch boards until you reach the last mark on the 1-by-4 inch board side rails. Attach one more 2-by-4 inch step to the bottom of the two side rails.
Attach the 1-inch angle iron to the bottom of the uppermost step with screws. Screw the angle iron to the side of the dock.