First off, I have to give credit where credit is due. This instructable is based heavily on base2john's great instructable entitled Holiday Toy Switch Adapting. In this instructable, I adapted those instructions to apply to a USB mouse.

Why a USB switch interface?

My wife is an itinerant teacher who works with kids with special needs and she had asked me to try to find a way to create one of these interfaces. I originally declined, as I had never done any soldering before, but after she was persistent, I decided to give it a try. Apparently it works great.

How much does it cost?

Excluding equipment costs (all set with my new soldering iron now), I put this together for between $10-15.

Step 1: Materials

Supplies
USB mouse (Picked up one from Walmart for $5. Brand was "Onn" or something like that)
3.5 mm audio jack (These come in a pack of 2)
Solder (Mine came with a kit I bought. It not sure the exact size but it was approximately 3/4 of a mm in diameter.)
6-10" of braided copper wire (Mine was salvaged from an old Dell power brick)

Equipment
Soldering Iron
Philips-head Screwdriver (Or a multi-bit)
1/4" drill bit (Use with screwdriver, or in a drill)

Step 2: Take Apart the Mouse

The mouse I bought was cheap, and it showed in the construction. The housing was held on with a single screw. Unscrew this screw and gently remove the housing. You should see something like the picture above.

NOTE: Be very careful removing the housing as with this brand the front it held down with small fragile plastic clips. I broke these clips and had to later glue the housing down with crazy glue. This problem is solved by using more expensive usb mouses which are held together with more screws rather than plastic clips. When I say more expensive, that is relative...I have since completed this project again with a $12 mouse and it was held together with screws...much easier to deal with...

Step 3: Solder Wires to Audio Jack

On the audio jack, there are 4 posts. For reference, the one long one that looks kind of like a clip of some sort will represent 12:00. The other 3 smaller posts with holes in them are found at 3:00, 6:00, and 9:00 respectfully. You will be soldering to the 3:00 and 9:00 posts. See the picture above.

NOTE: I apologize for the lack of clarity on the jack. I did not notice when taking the picture that the camera was focused on the soldering iron.

Step 4: Solder Your Wires to the Solder Points for the Switch

You will now solder the other end of your wires to the solder points for the left mouse button switch. You should really use a multimeter as recommended in the instructable I referenced at the top to see which solder points you should solder to. This will tell you which solder points will allow current to flow through when the switch is closed. I did not have a multimeter, so I guessed....wrongly. I had to take this apart again and resolder the wires. The red wire is correct in the picture, the white wire needs to be moved to the centre solder point.

Step 5: Your Mouse Should Now Look Like This...

...when fitted back into its casing...

Step 6: Drill Hole in Casing and Mount Audio Jack

Using your 1/4" drill bit, carefully drill a hole in the side of your mouse. To have good control over the bit here I placed my drill bit in my Picquic Screwdriver and drilled it by hand, but you can do it with a drill if you are careful.

Once you have your hole, unscrew the round, ring bit off of the jack, place the jack in the hole, and then screw the round, ring bit back on the jack.

See picture to see how it should look.

Step 7: You're Done

Replace the top of the casing and the screw that holds it on, and you're done! If you were more careful than me and did not break your mouse casing, your results should look quite professional. Good job!

One PS I would like to add...

I was very unconfident in my ability to accomplish this project until I checked out the videos at this link. They are very short. It links to a NASA Learning site that I saw linked in some other instructable. If you want to see the basics, this covers it in about 30 seconds!

http://radiojove.gsfc.nasa.gov/telescope/soldering.htm

Thanks for reading!