I should say something along the lines of "if you don't know how to do it, don't even try".
Have you tried reading the
FAQ stickied here? Or the
PSGroove guide? Have you even googled "PSFreedom" and tried to find the answers there? Or "how to use USBbreak"?
Short answer: find the latest PL3 payload to update it with, then follow the typical Jailbreak instructions. Get a normal USB drive and put your ".pkg" files on it (like a backup manager) and use the option under Games to install them.
EDIT: Oh right, downgrading. I don't know the details about that (because I've never cared) but some CFW are designed for 3.55 anyway, so there's no real need to downgrade.
Warning1: Sony will know about it the second your PS3 connects to the internet (at boot). There's currently no way around that without staying offline permanently.
Warning2: Jailbreak method is already more-or-less redunant as Custom Firmwares don't require you to Jailbreak it
every time you turn it on. Jailbreaking may have better compatability, but it's only a matter of time until the backup mangers designed for CFW iron out the bugs (they've only been out for a few days).
