first you must make a legal rip of a PS1 disc, make it a IMG file, i use Alcohol, and select CCD. let the disc rip and when its cmplete.
Find a GUI to popstation, i use IceTea, select the PSX file you want, this is the file you ripped before. Select the compression you want and where you wish the file to go, once this is done.
Cut or copy the folder containing the EBOOT.PBP into your GAME folder on your PSP, exit USB, go over to game where you would load any homebrew, select the PS1 game you placed, and play.
Check out the compatibility list also on this Forum to see if your game works or not. PLus i think NTSC works better than PAL games, but it is rumoured that this is fixed in 3.10 OE-A.
Hope it helps!!
Bookmarks