Place an extension ladder on firm, flat ground. If the ground is not level, put plywood squares under the shorter of the two ladder feet to steady it.
Secure the ladder to the ground with ropes tied to stakes. Press the stakes into the ground around the ladder.
Attach the top of the ladder to a roof rafter with a wire or rope. Tie the end around a nail and hammer it into a rafter.
Adjust the ladder so it is 3 feet or more from the top of the roof edge.
Climb the ladder and hold onto the top of the ladder as you step onto the roof.
Tap along the roof with the hammer. Listen for solid spots. These are the rafters.
Push the end of a pry bar between the top shingle and the one underneath it. Place the bracket in that spot and hammer it into the roof. If the nail goes in too easily, you missed the rafter. Move a few inches to the side and try again. Securing the bracket between shingle layers will hide the nail.
Measure from the bracket 16 to 24 inches to the side. Rafters are usually 16 inches apart in older homes, whereas newer homes are spaced 24 inches apart. Install another bracket once you find the rafter.
Lay a plank of wood across the brackets. Continue, placing one bracket every 4 feet apart. Step on the planks to keep your balance.