For the "front", I give a title like "Sonata K.545 - The turn". And then the "back" is a screenshot of a few bars (usually between 2 and 5 bars). If those bars happen to be towards the end of a line, I'll also include a screen shot of the clef (left-hand side), so I know the key!

When I am using the deck, on each card, I immediately hit the "space" key to show the back (because I am not using the deck to "memorize"). I practice the few bars on the screenshot, and then when I am bored (usually after a minute or two), I grade myself, by hitting the most relevant button (Again, Hard, Good, Easy), which gives the algorithm the info it needs to know when it should show me those bars to me again.

The deck works great on a PC, iPad or phone. Piano Practice in your pocket at all times!