Firstly you have to know which firmware your PS3 ia actually running.
Depending on that , if you have 3.55 and below it can be done.
If not ( 3.56 -- 3.6.X) there is no possible way to jailbreak or downgrade or execute any sort of hack with those firmwares.
(For the moment)
If its 3.55 or below:
Download your custom firmware (CFW) , for example Kmeow
Transfer that file to a usb stick, and put it at this dir
PS3/UPDATE/
Then go to system update and update as usual with the usb. You should now be running CFW.
Then you have to install a game loader from the usb, download for example multiman + Bdemu, by going to Install package files
Also you should install LV2 patch
And that is basically it.