The last thing you want to have happen during a roof-felt installation or when shingling a valley is to lose your footing. Particularly on roofs with an angle, or pitch, above 4-in-12, meaning a rise of 4 inches or more for each foot, you need to improve your footing and safety by installing roof cleats. A roof cleat is not a specialized piece of equipment. It's a regular 2-by-4-inch board at a full length of 8 feet attached to the plywood sheathing of the roof to serve as a foot support.
Measure the area of roof you plan to work on. Stack a sufficient number of 2-by-4-inch boards to place one cleat every 2 feet from just above the gutter to about 5 feet below the roof ridge. Place a handful of nails in your carpenter's belt pouch.
Climb the ladder to the roof. Lay the first 2-by-4 so it lies horizontally across the bottom of the roof just above the gutter. Nail the first cleat in place with 3-inch nails every 2 feet.
Bring the additional 2-by-4 boards up the ladder and temporarily lay them vertically on the roof to the side of the first cleat.
Move onto the roof. Support the toes of your work boots against the first cleat. Install the next cleat 2 feet above the first cleat. Continue to work your way up the roof, installing cleats that allow you easy access to areas where you need safe support.
Add additional cleats until the final cleat puts the ridge of the roof within comfortable reach of you and your hammer.