Step 1: Tools and Supplies
Flashlight (see note)
Male spade electrical connector
Heat shrink tubing (1/4 and 1/2 inches)
Double-stick mounting tape (the kind with the foam core)
1 inch Scope rings (make sure they will fit whatever base you will be using)
Soldering iron & solder
Knife or razor
Hole Punch (not pictured)
Note: I'm using a Miraclebeam 9 LED flashlight. It's about $3 at Fry's Electronics or Harbor Freight. It's cheap, bright and perfect for this specific modification. If you want to use another flashlight just make sure it's 1 inch in diameter, but you're on your own as far as modifying the switch.
Step 2: Modifying the Falshlight
Next you will need to cut 2 pieces of wire. They will need to be long enough to place you switch where ever you want it on you weapon once the light is mounted. Now solder 1 end of each wire to each of the contacts from the switch. You should solder the wire to the contact where the hole is, being careful to keep the soldered part contact as flat a possible. If you solder over the hole use a nail or awl to re-punch the hole.
Now pull the ends of the through the hole in the switch. Replace the contacts in the switch and press it back together. The solder will probable prevent you from fully seating the switch back together, just get it back together as good as possible. Though I didn't do it here, it's a good idea to put a bead of hot glue or epoxy around the wires where the come out of the switch. This will help it to keep from accidentally pulling out.
Next press the switch back into the switch holder (don't forget to replace the spring). Then punch a hole in the rubber cap and pull the wire through the hole. ( As you can see I originally cut a slit on the rubber cap but it eventually tore so, if possible, punch a hole in the cap so it won't tear easily) Then pull the wires through the back cap of the flashlight and press it all back together.
Step 3: Making Ther Pressure Switch
Now get a male spade connector and remove the plastic protection from around the spade portion. Then crimp it to the end of one of the wires. Now retrieve the small round contact from the flashlight switch (you did remember to keep it didn't you?) and solder it to the end of the other wire. (hint, solder the end of the wire into the little hole on the back of the contact)
Now we will use the mounting tape (the double stick kind with foam in between) to keep the contacts separate until pressure is applied. Stack 2 or 3 pieces of the mounting tape together. I needed 3 but you may need more or less depending on the thickness of the tape. Then use a hole punch to cut a hole in the stack of tape. To test to see if the tape is thick enough you can reassemble the flashlight and place the contacts on the tape. The spade will go on one side and the round contact goes on the other with the small raised portion going into hole in the tape. The flashlight SHOULD NOT light up when the contacts are held on to gently, but when more pressure is applied it should come on. Once you have the correct thickness peel the tape to expose the adhesive and stick the contacts onto the tape. Test the flashlight switch again.
Now trim the tape from around the switch to the shape and size desired. Then cut a piece of larger heat shrink tubing to go over the switch. I used 1/2 in but use whatever size fits. Heat it up to fit around the switch and test again. If everything went well you should have a fully functioning flashlight with a pressure switch.
You can give the switch a real professional look by dipping the pressure switch part in some Tool-Dip or liquid electrical tape and leaving over night to dry. (You can kind of see the difference in the opening picture)
Step 4: Mounting the Light
One problem I've had with these flashlights is the fact that the front lens is simply pressed into the housing and can come loose allowing the LED housing to lose contact with the battery pack. To remedy this just place a couple drops of glue on the edge of the lens where it makes contact with the flashlight housing.
Here you can see the flashlight mounted to a Kel-Tec SUB-2000 carbine.