Instructions
Open Notepad and click "File" and "Save As."
Enter the exact file name of your ROM file, but change the extension to ".cht." For example, if your game file is "Final Fantasy IV Advance.gba," your cheat file will be "Final Fantasy IV Advance.cht." You will create one cheat file for each game.
Copy and paste the cheat codes you wish to use into Notepad. Make sure each code takes up only one line. In addition, make sure there is a 1-line gap between each cheat code.
Add a header before each cheat code. The header should start with the version of the hardware you are using and end with a custom name of your choosing. For example, if you are using a code from the GameShark Version 3 and your cheat code is called "WalkThroughWalls," enter "gameshark_v3 WalkThroughWalls" for your header. For the Pro Action Replay, use "PAR" followed by the version number, similar to the GameShark formatting.
Copy your finished cheat file to the same directory as the ROM file.
Open gpSP and load your ROM. Go to the "Cheats/Misc" menu to enable or disable each cheat individually.