Hi There
There are a number of way to achieve the custom firmware install.
If you are on 3.55 ofw or lower then you can simply update to OFW 3.55 first then download an approved 3.55 CFW and install via usb stick update in service mode.
If you are above 3.55 OFW you can downgrade the console back to 3.55 PFW and then install 3.55 CFW.
Downgrade can be achieved using hardware flasher, PROGSKEET OR E3 FLASHER.
I recommend PROGSKEET as I have had greet success with it and I have tailored it to a few other hardware flashing applications.
E3 Flasher lacks compatibilty.
http://progskeet.com/Side note to mention is that when downgrading your console please check the date code on the sticker as any slim with a date code higher than 1A will not be able to be downgraded as the factory firmware was higher than 3.55, therefore when you attempt to load the firmware via downgrade the system fails to boot.
Check out the PS3devwiki site for all info regarding the system.
Also another important thing is that I have not yet seen any cfw install for ofw higher than 3.55 so I would advise against using that update as it will most likely brick your system. you have to downgrade to 3.55 ofw then install 3.55 cfw
