How do you make iso files work with 3.90 on a PSP?

Well first of all you will need custom firmware. If you have got it you need to press select then go down to divice drive i think it is and go accros to m33 device. Then make a folder in the root of your memory card called "ISO" in caps or it will not work. Then put the games in there wait till it goes onto your PSP then go to games memory stick then the game should be there.