Instructions
Look online for a ROM version of the game you want to edit. Make sure you own an actual physical copy of the game before you download the ROM because it is illegal otherwise. There are hardware stations that allow you to extract the ROM from the cartridges that you own, but these can be expensive.
Visit a ROM hacking website, such as Romhacking.net or Zophar.net and download the software that matches what you want to edit in the Game Boy game. Different utilities edit different portions of the game data, such as colors, images, music and level designs. Some only work with specific games.
Load the ROM you downloaded into the editing software and make the changes you want. Remember that you are working in the confines of an 8-bit console system, so don't try to overload the system with too many colors or large graphics. Once you've finished making changes, save the ROM file and note any errors that the utility reports which may make the game unplayable.
Download a Game Boy emulator. Since the ROM exists only as a file on your PC, you can't play it on your Game Boy without utilizing expensive hardware to write it back to a cartridge. You'll need an emulator to play the game with your changes. Popular emulators are Visual Boy Advanced and BGB.
Launch the emulator you downloaded and select the ROM that you saved with your editing changes. Enjoy your new custom game.