Connect the USB stick to the USB port furthest to the right of your console and start the console in recovery mode
Select option 6 (System Update), and follow the instructions that appear on the screen.
Once finished, enjoy the CFW 4.21.
Update: They have now released (via ps3-solution-pour-tous.doomby.com/blog/news-ps3/cfw-arch-4-21-finale-version-0-1-for-all-ps3.html) PS3 CFW Arch CEX 4.21 Final v0.1 for both PS3 Fat and Slim consoles (above) for those interested!
Stay tuned for more PS3 Hacks and PS3 CFW news, follow us on Twitter and be sure to drop by the PS3 Hacks and PS3 Custom Firmware Forums for the latest PlayStation 3 scene updates and homebrew releases!