well before you can get custom firmware you will need to downgrade to 1.50
have a read of these threads on a few ways this can be done.
either using the 3.50downgrade( you will have to upgrade first)
http://www.pspmod.com/forums/firmwar...6-27-07-a.html
or using Pandora's battery
what is this read here
http://www.pspmod.com/forums/hardwar...a-battery.html (What is a Pandora Battery?)
and how to make it read here
http://www.pspmod.com/forums/psp-gen...ew-method.html
hope this helps you out mate.