There are many references about how to use a Leitner box for spaced repetition and memorization. A plain and simple way to do this is to use an existing calendar and do easy division in your head.
Use five dividers to study new items, and one optional divider for new items that you have not studied yet. Divide today’s date in your head: “Hmm, it’s the 22nd today, is 22 divisible by 1, 2, 3, or 5? Yes, it’s divisble by 1 and 2.” Study the cards for dividers 1 and 2. At the end of the month, study all the cards that you’ve answered correctly from divider 5. If you fail to answer any of them, back to divider 1 they go.