Downgrading official firmware 3.90?

[picnic]

I've had my PSP for a few years and recently became interested in modding. Problem is, I have mine up to official firmware 3.90 and don't know if it's possible to downgrade it without a pandora's battery (which, to my knowledge, I need custom firmware to create, so I'm in a catch-22 there)

Is it possible to downgrade 3.90 to a point where I can begin installing custom firmware, and if so, where would I find it?