Leitner System are turned off by default on new decks. To turn it on find the switch that says "Leitner system" and, well, switch it on.
The first thing you will notice when you turn Leitner System on is that now there will be several tabs under the name of the deck: one for each box and one for all cards.
When Leitner is turned on for the first time all your cards will be in box 1, while box 2 and 3 will be empty. Naturally, this will change as you study them.
<aside> 🦊 The default number of boxes is 3, so this applies to all the examples in this guide. However, Muna PRO allows you to use up to 7 boxes.
Things are still basically the same as before: you pick one of the boxes — let's say Box 1 — click the button "Study Box", swap through the cards trying to remember the answers in the cards, and check your score at the end.
The only difference is that when you finish your study all the cards you correctly remembered the answer will be moved to the subsequent box (unless of course they are already in the last box), while the cards you failed to remember will be put back in Box 1 (unless, again, they are already there).
The idea is that you study Box 1 every day (or as often as possible), Box 2 every three days or so, and Box 3 every week (or way less often than the other 2 boxes).
<aside> 👉🏽 When you are using Leitner Boxes the new cards you create or import are always added to Box 1 (they are new so you have to study them more frequently). That's why the big purple plus button is not available when you are browsing the other boxes.
You can study multiple boxes simultaneously. In other words, you don't have to finish studying one box to start studying another.
You could, for instance, start studying Box 1, pause, and then start studying box 2. If you finish studying Box 2 the cards you failed to remember will be put back to Box 1, which means that, when you return to Box 1, you will now have to study all the cards that were left when you stopped plus the ones that came from Box 2.