how do you downgrade a psp with 2.71 official firmware???????

Well, first of all, I've been trying to find a way to downgrade MY PSP but out of all the questions I asked, they all said it's impossible…… So I went to my friends house to ask him to let me borrow his PSP, he has official firmware 2.71. I need to downgrade his PSP so i can get Custom FirmWare because I want to make a Pandora Battery using his PSP so that I can use it on my PSP. I heard that a Pandor Battery can only be made by using Custom FirmWare so that is why I want to downgrade my friends PSP so I can then upgrade it to a Custom FirmWare then i can make the Pandora Battery. My question is………………………………………………………………………………………….How do I Downgrade the Official FirmWared 2.71 PSP, and what is the lowest Official FireWare needed to update to a Custom FirmWare??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????