Once all the prerequisites are fulfilled, you can Rebirth by pressing the namesake button in the game. Here's how you can find it: Stay in the main game world in Flashpoint: Worlds Collide.

The Grandmaster Sprinkler costs 50 million Sheckles, the Grow a Garden game’s currency. To buy it, visit the Gear shop located between the Egg shop and the Crafting station in the game world.