The bare basics are this:
1. Copy the ProUpdate and FastRecovery folders to PSP/GAME when your PSPGo is connected by USB.
2. From the game menu on your PSP, run the ProUpdate. (Not the FastRecovery.)
3. Follow the on screen instructions to install.
4. Whenever you boot the PSP from it being completely off, it will start up in official firmware. Run FastRecovery to switch to the custom firmware.
After its installed, you can delete the ProUpdate folder. Keep the FastRecovery folder.
Did this with mine. Super easy.