This guide is pretty straight forward you should be able to follow it. I tried to be detailed as much as I can that it almost became like a dummy guide.

The files you need:

Yoshihiro's Decrypter V2: Download
UMDGenv4: Download
GEN-A / GEN 5.50 B2 CFW: For better compatibility and more chance of making the game work (you decide wether you want to upgrade or not) some say it works on M33 some do not.

1. Download all the needed files.
2. Open UMDGenv4 and
3. Select / Open the game you want to patch.

3.Click the + sign beside PSP_GAME to expand the folder
4.Click on the SYSDIR folder
5.Right click the EBOOT.BIN then choose “Extract Selected”
6.You can Extract the file directly on the root of your Memory Stick Pro Duo or on your desktop.

7.Copy the extracted EBOOT.BIN on the root of your memory stick if you did not extract it directly on your memory stick.

8.Copy GameZDecryptZ on E:\PSP\GAME\ (This is on your Memory Stick Pro Duo)
9.Run the decrypter from the XMB:

10.After running you should see the below:

11.Press X
12.Connect your PSP on your desktop or remove your memory stick then put it on your card reader (computer).

13. Double click DECRYPTOR folder from the root of your memory stick to open.

14. Drag the new decrypted file on UMDGenv4 Program just like below and select Yes to replace the existing EBOOT.BIN:

15. Click Save (You can either select: *.iso or *.cso):

16. Walaaaaah… Your game is ready, now put it on the ISO folder of your Memory Stick and Enjoy Playing…..

Note: The game I used is FIFA09 w/c is not really needed to be decrypted just for the sake of this tutorial as an example.