Step 0: Disassembling the Keyboard
Soundproofing your keyboard starts with taking it apart, and this can be tricky. Also, a couple of the mods (3 and 4) in this guide require the keycaps and switches to be removed. If you plan on doing them, it's best to do it right away, as they are much easier to remove from an assembled keyboard (if you have a version with knobs, it's also worth removing them).
Below is a video for beginners demonstrating how to remove keycaps and switches, along with the appropriate tool, called a switch/keycap puller, this process is quite simple.