After the wooden structure is put up, next comes the felt. This was done in several pieces, and more or less held in place by its own weight. After the felt, a covering - usually canvas - is placed on top of the felt. This is then lashed into place with two bands of rope around the ger.
That's basically it. The örh and other pieces still need to be put on, but this is the basic idea. The end result is a strong, sturdy structure, that can easily support the weight of a person on the roof. And you will notice (if you pay attention) that they do not use guy lines to stake the ger into the ground. There will often be a weight hanging from a rope attached to the toono inside, but otherwise, the ger is freestanding.