Obtain a piece of aluminum sheet metal. Use a straight edge and a utility knife to score the sheet at a width of 8 inches. Bend the sheet metal back and forth until it snaps. Then, bend a 2-inch lip along the edge. Use a rubber mallet and a wooden block to carefully bend the lip to a 90-degree angle.
Observe the problem areas where rain is pouring from the room. Cut the rain diverter down to 1 foot longer than the length of the problem area. This extra length ensures that splashing does not become a problem where the rain is diverted. Most rain diverters are made of sheet metal. Use a utility knife to score the metal and bend it back and forth until it breaks.
Loosen the shingles where the diverter will be installed, using a pry bar. Set the sheet in place so that it is at a slight angle, to ensure water flows downward. A downward slope of 1 inch every 6 feet is sufficient.
Slide the long part of the piece under the shingles and secure it in place with roofing nails and a hammer. The lip should be exposed and facing up and out.
Apply roofing cement to the nail heads so that the shingles stick in place.
Spray water at the installation with a garden hose to see how it flows.