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!
If you do, I would check out one of the more established PS3 CFW's... specifically either Rebug, Rogero or ErmaC as some of these others are more experimental (for people with hardware flashers who don't mind taking a risk with their console).
As for the procedure, it is covered fairly extensively in the Rebug and Rogero threads... if you need links I can dig them up.
Following up on their previous release, French developers Dev-Team-SPT / ARCH have released PS3 CFW Arch CEX 4.30 Rev 2.00 and 4.00 Custom Firmware alongside a MultiMAN Stealth version for those with flashers who don't mind experimenting.
To quote, roughly translated: Revision 2.00 CFW Update to 4.30 released with CEX ARCH fixes and additions detailed below and a version of MultiMAN v04.13.00 Stealth.
As a reminder, a version of MultiMAN Stealth allows you to play backups of your games when connecting to PSN but you will appear as extended spirited play on the game demo version of JAP-NPJA90067 instead that you see on MultiMAN. A big thanks to Deank manager for Great About!
NOTE => For the moment it is too dangerous to release a 4.31 CFW because too unstable (a Core_os re-compiled on different CFW / OFW does not mix well with our ps3 as we have seen recently). I prefer the stability brickage massive!
Installation directory MultiMAN Stealth => dev_hdd0 / game / NPJA90067.
ARCH CFW 4.30 rev 2.00 (CEX PS3 only)
Add: compatible PS3 BD player off (must be used in MultiMAN latest version available for your backups run games)
Add: Rebug Package Manager
Add: Patched disable Cinanvia
Fixed Core_OS (more stability)
Optional: Change coldboot (all public)
Optional: Police original writing
Known bugs: problem classification in game menu and XMB displaying the home PS3 demo mode (correction to come)
MultiMAN Stealth Version:
Added a wave.divx
Theme modified original
Stealth Version => game demo version of JAP NPJA90067
All that is present on our site (CFW / patch / fix / application) is for limited personal use of the family circle and their installation or use is at your own risk and we disclaim any responsibility!
Download: http://www.filefactory.com/file/5kplnwi38pgp/CFW_CEX_ARCH_4.30_rev_2.00.rar / http://www.filefactory.com/file/1tczzfytvyb5/n/Multiman_Stealth_version_NPJA90067_rar (THX DeanK for manager !) / http://uploaded.net/file/9ppj6bub / http://www.filefactory.com/file/2ygn8e80p9o3/n/CFW_CEX_ARCH_4_30_v4_00_update_rar (Mirror - via ps3-solution-pour-tous.doomby.com/blog/news-ps3/cfw-cex-arch-4-30-update-rev3-00.html)