1 - Find a friend that has custom firmware and ask him to make a Pandora's Battery which downgrades any PSP.
2 - Buy a Modded Battery off a place like Divineo and find the pandora files online.
After you choose from one of those options, follow instructions from online sites (google is your friend) on where to put the files on your memory stick. Then put the modded battery in your PSP and after you downgrade it, you should have a modded PSP ready to boot any kind of file (unless its a slim model)
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