Step 1: Tools and Parts
We will program the Teensy to send a unique keystroke combination, Meta-Alt-Control-W, that will be intercepted on the computer - for Windows you can use AutoHotkey, for Mac you can use QuickSilver, and for Linux you can set it up directly in the Window Manager's preferences using xev or keytouch.
You'll need to do a little scripting, but the Lifehacker links I used above have many common script libraries to get you started.
Finally, you'll need an Easy Button.
For tools you'll need a P0 or P1 philips screwdriver, a soldering iron, and about a foot of hookup wire. A coping saw or hacksaw is helpful for cutting the hole for the usb port into the side of the button, but a pair of pliers will do just fine with the relatively soft plastic, it will just look a bit more ragged. Not pictured are an xacto knife or small flat screwdriver, and a wire cutter/stripper.
Disclaimer: I'm not affiliated with Microsoft, Apple, PJRC, Adafruit, Lifehacker, Quicksilver, Autohotkey, nor Staples. I just like their products and used them in this Instructable.
Any soldering or other hacking is at your own risk.