Cut the coil stock to length. If you need a piece of flashing longer than the throat of your sheet metal brake, you'll have to make it in sections.
Mark the depth of the hem on the coil stock. Most hems are between 3/8 inch and 3/4 inch deep, with 1/2 inch being typical.
Place the coil stock in the brake, with the hem protruding from the throat. Move the lever to clamp the throat shut.
Bend the coil stock beyond 90 degrees to begin the hemming process. Bend as far as possible --- usually about 120 degrees.
Remove the work piece from the brake. Place the bend made in the previous step against the brake's hinge. Pull up on the bending handle to complete the hem.