Go into the Player Market or Shop. Find a Cosmetic Item that says 100% Yummy Cake. It can be a Cake, Pie, Watermelon or even a Turkey Dinner, but they'll say 100% Yummy Cake. If you have a friend who's willing to sell you one for cheap, you can get 150 Diamonds for verifying your email address in the Quest list. Otherwise you'll need to invest $5.00 (US) to buy the cheapest Diamond Pack which will give you enough to get a Cake Item & will allow you to get more Diamonds from quests.
Once you have your Cake Item, go play Co-op or VS. You, plus someone else in your game will need to have a Cake Item equipped for it to count as a share. You can equip up to 3 Cake Items per player. Theoretically, 18 cakes can be shared in one game, you could get this in 28 games...or 248.
Updated since you no longer recieve 500 Diamonds when you begin.