Pretty simple... Just play and win one of the Daily Challenges in the game. Doing one of the "Easy" challenges is obviously the quickest route to a win.
The challenge types are:
Classic
Detonation
Flags
Taps
Treasure Hunt
Personally, I find "Detonation" to be the easiest of the challenges, and "Treasure Hunt" the toughest, but YMMV.
Good Luck!