You win Coins for every Daily Challenge you complete. The Bronze, Silver and Gold badges require you to have enough coins. For completing all three of one days challenges, you get bonus 150 coins. You can also go back throughout the month to play challenges from earlier days, but you cannot go back to previous months.
Importantly, you must make sure that the game is properly registering the actual date. To ensure that this is the case, I recommend that you completely quit out of the game each day and reopen it the next day otherwise it's possible that the game will register that you're playing a challenge for a given day on the day before.
- Bronze badge requires 2,000 coins monthly.
- Silver Badge requires 6,000 coins monthly.
- Gold Badge requires 10,000 coins monthly.
The monthly badges will stack, so you should start off by getting a gold medal for the first eight months, unlocking "Go for the Gold." For the next two months you only need to earn silver medals, bringing your total number of silver medals up to ten, unlocking "Go for the Silver." Finally, you just need bronze medals for the final two months, giving you twelve total and unlocking "Go for the Bronze." It's certainly not a challenging achievement, but there's no way to speed it up. You really do have to focus on it for a year.
You can track your current coins and badges of the month from the bottom of the screen when in Daily Challenge menu. It looks like this: