Instructions
Research the "Burrow" ability at the Hatchery. This allows several of your units, including Hydralisks, to burrow underground; while they are burrowed, other units can pass over their location unobstructed. The stacking bug takes advantage of this behavior.
Select a Hydralisk, then hold down the "Ctrl" key and press "1." This assigns the Hydralisk to your first control group; pressing the "1" key now will select the Hydralisk.
Select all the other Hydralisks you want to stack and very quickly press the following keys and key combinations in succession: "U", "Ctrl" and "1," "1" and finally "U." Pressing these keys quickly enough will take some practice, but once you've done it, you'll have a control group of Hydralisks the game erroneously thinks are burrowed -- although they still appear above ground and can move around. Order them to move normally and you will see that they can occupy the same space as each other.