The custom power-up you can add in the Forge will greatly make or break your game (depending on how many you put in your map and how fast they respawn). Since you can only adjust the custom power-up just before a match, be sure to jot down what effects you are looking for in a custom game.

Custom power-ups can only be used in Custom Games. Press BLUE to edit the match options in a Custom Game. Go into Weapons and Vehicles, then locate the Custom Power-up Traits. Have fun!