you have to have a pandoras battery and magic mem stick in order to downgrade your psp. or you can buy another motherboard for your psp with custom firmware on it, or another firmware that you are able to downgrade from.
use the easy pandora installer guide here:
http://www.pspbrew.com/forum/viewtopic.php?t=9572
its easy to use and you can easily revert your battery and mem stick back to normal. all you need is another psp with custom firmware on it in order to make the battery and mem stick.