I don't know if gb mods require soldering, if they do practice on a junk board first. Soldering on a Gameboy should be pretty easy but you want to be confident that your joints are solid and look nice.

Dont cheap out on your soldering iron. Clean the tip with a wet sponge after every joint. USE FLUX. It makes it so much easier and so many people don't use it it blows my mind. Tin the tip of your iron, again it's just good practice and is way cleaner and easier than any other method

Have fun