Lift the shingle tabs that are centered over the porch stoop by gently pushing the flat end of a pry bar under each tab. If the asphalt sealant holding the courses of shingles together won't release, apply more force to the pry bar by punching the back of the bar using your palm or tapping the bar lightly with a hammer. Work in temperatures of about 40 to 70 degrees F.
Apply a 1/4-in. thick bead of clear silicone caulk on the underside of the drip edge, lift the shingle tabs up slightly and slide the diverter under the tabs. Slide one end of the diverter tight to the bottom of the shingle tabs and leave a 1-in. exposure at the other end to create a drainage pitch.
If your house lacks gutters, rain that's pouring off the roof will give you a good soaking when you come and go. In winter, melting snow creates a slip-and-fall hazard when it drips and freezes on porch stoops. Avoid those problems by taking 25 minutes and using the techniques shown in Photos 1 – 3 to install a rain diverter.
Purchase metal D-style drip edge (ordinarily used as roof edge flashing) and install it upside down as a diverter. Drip edge comes in 10-ft. lengths and is usually available in white, brown and unpainted aluminum. Cut it to length, equaling the width of your stoop. Also buy a 10-oz. tube of clear silicone caulk for use as an adhesive and sealant.
Work carefully to break the bond to release the shingle tabs (Photo 1). Work from a stable ladder centered on the doorway and release only those tabs along the third course of shingles up from the eave. To avoid breaking the tabs on older, brittle shingles, lift them only 2 to 3 in. On newer roofs where the shingles are more pliable, fasten the diverter in place using both silicone (Photo 2) and roofing nails. Space the nails every 2 ft. under the shingle tabs. Caulk the nailheads with silicone.