Next you need to find the best spot to put the switch. It should be 1/4 to 1/3 of the way from the pivot bolt/pin hole to where the lever pulls the cable but there is some room for flexibility here. Make sure that where ever you place the hole, the switch will contact the brake lever when it is released. On some switches , there is enough room next to the reach adjustment screw to put the switch or there is no reach adjustment screw and you will not need to do the filing in this step. In other cases (first picture), you may want to use this screw's hole for the switch hole and you will not need to do the filing in this step . In still other cases (second picture), the reach adjustment screw will be in the way and it will need to be permanently removed and any protruding material around the threaded hole will need to be removed. First clamp the body in the vise and if needed, use a half round metal file to remove any protruding material around the brake reach adjustment screw hole until the surface is smooth and relatively flat. Then use some sandpaper to make the surface smooth. Next, mark where the switch hole will be drilled using a pencil. It should centered between the sides of the body and at least 1/4" away from the center of the brake reach adjustment hole. Any closer will risk the the two holes merging into each other. In the photo, I marked it 1/4" away from the brake reach hole. Once you have the spot use an auto punch or center punch to make a depression so the drill bit will not wander when you drill the hole.