Find a ROM you like, the first thing you should do is make sure it is COMPATIBLE with your phone. If you find a ROM that is for AT&T and you have the International model, don't be an idiot and flash it.S3 ROMs don't work on S4 models and vice versa.So you found the ROM you want to try, so go ahead and download it.Connect your USB cable to your phone. View the folders on your phone. I like to make a folder called ROMs on my phone and transfer it in there.After you transfer it, open my toolkit and press Boot into Recovery. When you are in TWRP, MAKE A BACKUP!After you make a backup AKA a Nandroid, click Wipe. Wipe Cache, Dalvik, Factory Reset...