In this video, This Old House general contractor Tom Silva shows how to fabricate custom-fit aluminum window flashing.
1. Cut a piece of 8-inch-wide aluminum flashing to length using aviation snips; you could also slice through it with a sharp utility knife.
2. Measure for the leading edge of the flashing to determine how far it'll hang down in front of the window trim.
3. Use a metal brake or homemade bending brake to bend the leading edge of the flashing passed 90 degrees.
4. Make a gauge block from scrap wood to represent the thickness of the window casing and trim.
5. Slide the flashing into the brake, using the gauge block to position it at the proper depth.
6. Bend the flashing to create the wide flange that extends up the wall.
7. If using a homemade bending jig, set the 2x4 tapping block on top of the bent flashing and strike hit it with a hammer before removing it from the jig.