A typical spaced repetition system includes these key components:

• A schedule for review of information. Typical systems involve going over information after an hour, then a day, then every other day, then weekly, then fortnightly, then monthly, then every six months, then yearly. Guess correctly and the information moves to the next level and is reviewed less often. Guess incorrectly and it moves down a level and is reviewed more often.
  
• A means of storing and organising information. Flashcards or spaced repetition software (such as Anki and SuperMemo) are the most common options. The software has the obvious advantage of requiring little effort to maintain, and of having an inbuilt repetition schedule. Anecdotal evidence suggests that writing information out on flashcards contributes to the learning process.
  
• A metric for tracking progress. Spaced repetition systems work best if they include built-in positive reinforcement. This is why learning programs like Duolingo and Memrise incorporate a points system, daily goals, leaderboards and so on. Tracking progress gives us a sense of progression and improvement.
  
• Set duration for review sessions. If we practice for too long, our attention wanes and we retain decreasing amounts of information. Likewise, a session needs to be long enough to ensure focused immersion. A typical recommendation is no more than 30 minutes, with a break before any other review sessions.