This achievement is awarded when you spill a total of 1,000,000 drops in the game, across all attempts of all levels in all of your playthroughs. This achievement is a little ridiculous considering how little content there is in the game. Since only the water (blue) drops count, it will take around 30 hours to reach 1 million drops (roughly 10 drops fall every second).
With that said, there are two options, depending if your Windows Phone device has the option to set the screen time-out to "Never" or not. To check, go to Settings>Lock Screen>Screen times out after.
If you CAN select "Never", you are in good shape. This means that you can grind this achievement by leaving the game running in a level while you are sleeping or doing anything else. For this step specifically, make sure your device is plugged into the wall to charge.
If you CANT select "Never" and your maximum time-out is 3 or 5 minutes, this achievement is slightly more frustrating and time consuming to you. Set the time-out to the maximum time. You will need to start up a new game at Level 1, then let the drops spill while you passively touch the screen every so often to keep your screen from timing out. After you have spilled as much as you want to in a single session, you must finish the level and hit "Save and Continue" to lock-in your progress. Continue your spilling sessions through the levels until the achievement unlocks.
After your period of grinding, you must hit "Save and Continue" to save your progress from that session. If you accidentally hit the Bing or Start buttons, or let your screen lock, you will lose ALL grinding progress, as this game does not have the fast app resume capability. The achievement will unlock mid-level whenever 1 million drops is reached. Keep all of this in mind when spilling those drops!
*GLITCH WARNING* This achievement is a tad glitchy. For some reason, some folks just cant get the achievement to unlock. Techniques that work for some people don't work for others. Some claim the drops can be done across multiple save files, while others claim it can only be done on one. You may even need to completely reinstall the game to earn this. The best method to go about earning this achievement is to reinstall the entire game after earning all of the other achievements but this one. Then start your grinding fresh on a new save file. This should prevent it from glitching on you.