Instructions
Obtain Netherstone. To do this, you must mine it directly from the Nether. This is accessed through a 2-by-3 obsidian frame lit on fire.
Place the Netherstone into one of your equipment slots. This is the row of squares below your inventory.
Press the key bound to the equipment slot. For example, your last equipment slot is bound to the "9" key.
Right-click anywhere to place the Netherstone on the ground.