3. After playing you can go to current challenges, "Your Challenges". From there you input the scores of the game. Both users DO NOT have to input the score, although by clicking in the same button the loser must confirm that the score is correct. You also have the option to post a screenshot, for proof. I don't think any of us will need this, but it's still a nice feature.
4. Now that your score has been input and your opponent confirmed the score you are all set. The leaderboard will update and you are ready to find a new challenger.