Summarize this article:

Firmly sit on the canvas between the first and second sets of legs, then slide down the cot and firmly sit between the second and third sets.  If you have never put the cot together before, the canvas will be difficult to maneuver. Sitting on the canvas stretches it out. It is recommended that you do this before finishing the frame to make the remaining part of the procedure easier. Note that you should be careful while doing this, however, since the cot can collapse at this point if too much weight is applied. Walk around the perimeter of the cot while firmly pressing into the canvas with closed fists.  Press down across the entire perimeter of the canvas. Punch the canvas down the entire lengthwise center, as well. You can do this instead of or in addition to sitting on the canvas. Either way, this action should help stretch the material and make it easier to maneuver.

Summary:
Sit on the canvas. Punch the canvas down.