Two common methods for plying yarn are the Andean (2 ply) and Navajo (3 ply) techniques.

Andean is for 2 ply, Navajo is for 3 ply.

Andean Technique shown here

http://www.mielkesfarm.com/andean.htm

You can also ply from a center pull ball by using both ends of the ball or "cake" at the same time. Put the ball in container to keep it from rolling all over the room.

Navajo Technique shown here

Here is Navajo Technique on a spinning wheel