There are different methods for memo like roman rooms, journey method, letter pairs, visual, audio... I think the easiest for me is letter pairs so that's what I use and I just make up a word for it. The common system for lettering each sticker is called
Speffz. I actually use my own system although I haven't done much memo and I'm always dreaming about a better method for personal use. (Like I said, I have bad memory with the exception of music so I always thought if I could transform each sticker to its own tone or something along those lines I could memorize a sequence of notes better than letters or numbers.) If you like images, you can recall an image also using the same letter pair idea. The good thing about doing it in pairs is that you can very easily tell whether you have parity or not.