Downgrade 2.60 using a 3.90 M33?

Hello everyone :thumbup:
i promised a friend i'd downgrade her PSP for her, and i need a bit of help.
i'm aware that you need a 1.5 PSP to downgrade another 2.6 or 2.5 PSP (yes, i have already googled it, so i'm not being a lazy bum, i promise :))...my question is: is there a way to downgrade her 2.60 using my 3.90-M33? if so, can someone please provide a 'TOTAL-idiots-guide-without-too-much-technical-wording' ? :confused: yes i admit i'm a total noob...and im VERY worried about bricking her PSP because i don't have access to a pandora's battery...:( all of the websites i've seen gave all those warnings that everything was in 'beta' version but i didn't know how recent this was, because i couldn't find any dates. so is everything still in beta or is it all tested and fine? (with the small potential risk, i do understand that) the most i've ever done is upgrade my PSP (which was already in custom firmware) up to 3.90-M33, but i've never started from scratch.
P.S I've cheked her PSP and it's not a TA-08 (or soemthing similar, whatever it was, you know that motherboard that would brick if you ied to downgrade) so i'm cool with that bit.

all help is GREATLY appreciated (you'll save me looking like an idiot going 'i cuoldn't do it' to her :p)
thanx :D

You can pandorize you battery and a mem stick. After that, you can restore your battery back to normal.

Here's a giude http://www.dcemu.co.uk/vbulletin/showthread.php?t=86997