Many sources disagree about the exact way to make a Mind Eraser, but they all agree on this:

The soda can be either a lemon-lime soda such as Sprite or 7-Up, or plain seltzer.
Serve either in an old-fashioned glass filled with ice, or a shot glass (clearly, no ice).

Pour the Kahlua in first, then carefully add the vodka and top off with the soda. If you do this correctly, there will be three layers in the glass. Serve with a straw, and drink all at once, so the Kahlua is ingested first and then "chased" by the vodka and soda.