For this glitch you'll need a level of enchanting that will give you at least -25% magika useage to whatever type of magic you normally use, 4 grand soul gems, a ring, a necklace, a no enchanted helmet of your choosing, and the worn dark brotherhood cowl. Enchant all these items. Put on the worn dark brotherhood cowl first before putting on your other helmet, You should notice that you are wearing both pieces of head gear at the same time. Put on the ring and necklace then use a spell. Your magika bar wont appear and your can use the spell as much as you want. Normally you'd only be able to have around 75% -magika usage but the glitched nature of the worn dark brotherhood cowl negates this, allowing for -100% magika usage.

You can normally get -100% magicka usage in a school (two with the perk).