I have posted sock bun tutorials before. Click on the pictures below if you want instructions.
For a sock bun form, get an old sock you don't need anymore. The closer to the hair color the better in case any of it shows out. Cut out the toe.
Put the hair into a ponytail. Insert the ponytail through the middle of the sock. Pull it up to the head. Straighten the tail hair, and hold it so it is tight together like a stick. (I didn't do this in the picture but figured it out later on.) Bring the sock down to where the shortest strands of hair are.
Here is a video: