How can I downgrade psp Fw 4.05?

Ok, here's the thing, I just fixed a psp (fat, old one) and it's firmware 4.05, I wanna downgrade it.

I'm not new to the whole psp hacking scene, I had downgraded and installed custom Fw on 2 PSPs before, but I haven't been on the "scene" for over a year now.

I've read the pandora/magic mem thing, but there are so many different variations and some are outdated.

Can anyone help with the most recent way of psp downgrade?