Download John GBA lite or the paid version (ad-free) from the Google play store.
Go to www.coolrom.com in your mobile browser.
Search and download any GBA game of your choice. Example.... Final Fantasy Tactics Advance
Once a game rom has been downloaded, open John GBA. Note that all game will be detected automatically. Once app is launched pick any game you have downloaded, and BOOM! you will be playing your favorite GBA titles on you android device!