Step By Step - 4.20 JB

First, download 4.20 Beta v0.1: http://www.mediafire.com/download.php?wx57qazw9c4kgav

The reverter version is to go back to original firmware.

Okay so once you've done that, get a flash drive. 
Make a folder inside it called " PS3 " 
Make another folder inside " PS3 " called " UPDATE "
Then drag the PS3UPDAT.PUP (what you downloaded) inside " UPDATE " .
P.S ... Case Sensitive, type in CAPS and don't include the quotations..

Anyways, it should be like this - 

PS3 > UPDATE > PS3UPDAT.PUP

Now you're ready to jailbreak.

Go to your ps3 and make sure it's off(stand by: the light is RED)

Now you hold the power button down on your ps3, it'll beep once then once more, and it'll turn off. 

Let go for a second, then hold down the power button again it'll beep once, twice, THEN let go.
If done right you should be in recovery mode, correct?

(Make sure your flash drive is plugged into the ps3.)
Goto #6 "System Update"
and follow that through, it'll update all the way through and eventually you'll make it back to XMB (PS3 Menu)

And you'll notice you're in something called demonstration mode.. 
You'll have to put in the code (4 8 0? 1 or 0 0 0 0) everytime you turn on your ps3, but that's not to hard to do. Very simple to be quite honest. Just make sure you're hovering over "security settings | under "Settings", In order for the code entering process to appear, you have to press L1 + R1 + Select + Square . All in that order, just make sure you're holding down the buttons while pressing the others.
And, boom. You're on 4.20 v.1 Jailbreak!