The first thing you have to consider is that you have to have the files of the games stored somewhere on your device. The elements compatible with John GBA Lite are the GBA files which you can find easily with a quick search online. Once you've downloaded your favorite games, the app will automatically recognize them and will show a list of all the elements you can play. On this list, you can see the size of the game, the last time you opened it, and the exact frame where you saved the game. One of the best things about this tool is that it doesn't matter if you can't save within the actual game - the app itself will maintain your progress so you can pick up wherever you leave off.
Another one of the great things about GBA Lite is that it offers a huge menu of options, completely customizable that you can use to edit any element regarding the image, sound, keys, and other elements to help you enjoy the games the way you want.