Figure out where you will place the ladder on the roof. Open the legs of the ladder and stand it on the roof.
Mark around one of the ladder's legs on the roof with a grease pencil. Move the leg away from where you just make the mark.
Place the strip of safety wood, also called "roof jack," on the grease pencil marking. Screw wood screws through each corner of the safety wood using a Phillips screwdriver.
Place one of the legs of the ladder on the safety wood. Move the leg up against the ridge at one end of the wood.
Remove the leg from the wood when you are done using the ladder. Unscrew the wood screws and remove the safety wood from the roof. Apply silicone sealant to the screw holes that were made in the roof to seal them from the elements.