1. Download Yoshihiro's Decrypter
2. Unrar it
3. Copy the files in the PSP/GAME/ folder
4. Pick a ISO file
5. Download UMDGEN
6. Open your ISO with UMDGEN and browse to PSP_GAME/SYSDIR
7. Right click to EBOOT.BIN
8. Extract it somewhere
9. Copy it to the root of your memory stick
10. Unplug the PSP and load Yoshihiro's Decrypter
11. Press X and wait a few seconds
12. When the PSP has restarted, re-plug the USB cable and connect it to your PC
13. In your memory stick root, there should be a new folder called "DECRYPTOR", and inside it there should be a file called EBOOT.BIN. Take it and copy it somewhere safe. Do not mix it with the EBOOT.BIN you previously extracted from the ISO.
14. Open UMDGEN again, load again your ISO, go again to PSP_GAME/SYSDIR, right-click EBOOT.BIN and select delete
15. Right-click somewhere in the SYSDIR folder and choose Add -> Existing File(s)..., and browse to the EBOOT.BIN you found in the DECRYPTOR folder
16. Click the "Save" button in the toolbar, select "Uncompressed (*.iso)", name it and wait a few minutes.
17. Copy the newly-created .iso file into the ISO folder of your PSP
18. Unplug and start it. It should work. If it doesn't, try updating your custom firmware and/or changing drivers (this can be done by pressing select and by scrolling to the "UMD ISO MODE" voice. If the menu doesn't show up, power off your PSP, keep the R trigger pressed and power it on again. A recovery menu should appear. Go to Configuration, and look for the voice "Use VshMenu". Change its parameter to "VSHMENU", then go back and reboot the PSP. Now the menu should appear. Try changing the drivers, it may work. If it doesn't, just pray that someone smarter than us comes up with a solution.
EDIT: Damn it, djricekcn, you beat me to it.
2. Unrar it
3. Copy the files in the PSP/GAME/ folder
4. Pick a ISO file
5. Download UMDGEN
6. Open your ISO with UMDGEN and browse to PSP_GAME/SYSDIR
7. Right click to EBOOT.BIN
8. Extract it somewhere
9. Copy it to the root of your memory stick
10. Unplug the PSP and load Yoshihiro's Decrypter
11. Press X and wait a few seconds
12. When the PSP has restarted, re-plug the USB cable and connect it to your PC
13. In your memory stick root, there should be a new folder called "DECRYPTOR", and inside it there should be a file called EBOOT.BIN. Take it and copy it somewhere safe. Do not mix it with the EBOOT.BIN you previously extracted from the ISO.
14. Open UMDGEN again, load again your ISO, go again to PSP_GAME/SYSDIR, right-click EBOOT.BIN and select delete
15. Right-click somewhere in the SYSDIR folder and choose Add -> Existing File(s)..., and browse to the EBOOT.BIN you found in the DECRYPTOR folder
16. Click the "Save" button in the toolbar, select "Uncompressed (*.iso)", name it and wait a few minutes.
17. Copy the newly-created .iso file into the ISO folder of your PSP
18. Unplug and start it. It should work. If it doesn't, try updating your custom firmware and/or changing drivers (this can be done by pressing select and by scrolling to the "UMD ISO MODE" voice. If the menu doesn't show up, power off your PSP, keep the R trigger pressed and power it on again. A recovery menu should appear. Go to Configuration, and look for the voice "Use VshMenu". Change its parameter to "VSHMENU", then go back and reboot the PSP. Now the menu should appear. Try changing the drivers, it may work. If it doesn't, just pray that someone smarter than us comes up with a solution.
EDIT: Damn it, djricekcn, you beat me to it.