The actual method is to put down the "Region Boundary" bricks at the corners of each minigame. They need to be the same color as the "minigame color" that you selected in the slayer window.
They need to form a box around anything relating to the minigame. You only need two, for example, one at the furthest North East, at the bottom, and one at the furthest South West, at the top. Though you can add more to be safe, putting one in every corner will also help if you're still unsure how to use them.