Measure the top edge of the exterior head -- top -- casing using a tape measure. Add a 1/2 inch to this measurement, which is the length required for the drip cap. It will allow the drip cap to extend 1/4 inch past the ends of the head casing.
Lay the drip cap flat on the table of the miter saw. Measure and mark the drip cap to the length that you calculated in Step 1 using a tape measure and pencil. Saw the drip cap to length using the miter saw.
Cut the tip off from the window and door caulk tube using the built-in cutter on the caulk gun. Puncture the seal inside the tip with the seal puncture tool on the caulk gun. Insert the caulk tube tip-first into the caulk gun.
Apply a generous bead of caulk along the entire top edge of the head casing using the caulk gun.
Center the drip cap over the top edge of the head casing. Hammer finish nails every 4 to 6 inches through the top of the drip cap into the head casing. Dab a small spot of caulk over each nail head to seal it.