Instructions
Select a Zerg drone unit, Protoss Probe unit or Terran SCV unit. Depending on which race you select to play the game, you will have worker units that construct buildings and defenses to protect your base.
Press and hold the "Control" and "1" buttons on your keyboard together. This will activate the stacking mode in your game.
Click on the defense building that you want to stack. For Protoss, you will need to build defense canons. For Terran, the defense buildings are the bunkers. For Zerg, sunken (for ground attacks) and spore (for air) colonies are the defense buildings. Continue to hold the "Control" and "1" buttons.
Click on the area where you want to build the defense units. Repeat Steps 2 and 3 for additional defense buildings that you can now stack on top of one another or build within close proximity of each other.