If you don't feel like reading, you can watch this video. Also, I played a little demonstration in the video.
Step 1: Materials
Here's what you need to make this.
Small Project Box
Soldering Iron (optional)
A note about the button. I got mine at RadioShack. MAKE SURE that the button is usually open. That means that when you aren't pressing it, it's off. If on the back of the packaging the button came in it says "Usually Closed" DO NOT buy that button!
Step 2: Attach Wires to Guitar Cable
Unscrew the metal casing from one end of the guitar cable. Take off any plastic or rubble protection inside it. Wrap two wires around the things highlighted in red on the picture. To make sure you did it right, plug the cable into a guitar, play a note, and touch the two wires together, if the sound stops, you're doing great so far! If not, make sure you have the wires in the right places.
Step 3: Put the Cable Back Together
Wrap anything that was originally covered in rubber or plastic in electrical tape. Run the two wires you attached under the metal casing and slide the casing back on. This part can be a bit tricky. If the wires don't fit, try thinner wires.
Step 4: Make the Box
Drill a hole in the top of your project box whatever size the button's packaging says to drill. Mine was 3/8th inches. Also drill a hole in the side of the box. Run the wires through the hole on the side and into the box. Put the button into the hole, then either twist, or twist and solder the wires onto separate pins on the button. Put the top back on the box and screw it back together.
Step 5: Attach It All to Your Guitar!
Now, using duct tape or velcro or something, attach the box on to a place on the guitar you can easily reach. Plug the cable into the guitar and try it out! It's a little tricky at first, but with practice, it'll sound good!
Try moving it to different places! Putting it in the lower left makes it easy to hit with your arm!
If you really like the killswitch and think you're ready to actually install one, check out this Instructable by pandaman0529!