Creep stacking is the act of increasing the number of creeps by pulling the creeps out from their camp at the right time but it doesn't require your own creeps like the pulling creeps technique.
Each minutes, a new natural creeps will respawn if the camp is empty. By making it "looks like empty", a new creeps will respawn. This will double your money when your farm during creeps spawning. To do this, attack the creeps between 52-53 seconds (see picture).
Run away from the creeps. Avoid any allies hero or empty camp because sometimes when you pulling a creeps, you have vision at the empty camp that will prevent creeps respawn when the time at 00 seconds.
To succesfully stack the creeps, the time must hit a new minutes (e.g. refer picture) before the creeps reaching their camp.
Go back to creeps' camp and check if the new creeps respawn. If not try again, with few pratices you will be a pro in stacking the creeps.
You can stacks several times. Like in this picture, it has been stacked for two times.