There's a many different ways to do it, and how it's done depends on personal preference.
In this variation, the Electrician will leave one of the grounding conductors long. He'll install a ground screw in the threaded hole in the back of the box, and loop the long conductor around the screw around the middle of the conductor. Then he'll collect all the grounding conductors in the box, and twist them together with the remainder of the long grounding conductor. Once twisted, he'll use either a crimp or twist-on wire connector to "permanently" connect the conductors together.
Another option is to gather all the grounding conductors together, and twist them together with an extra bit of grounding conductor. Again using a crimp or twist-on wire connector to "permanently" connect the conductors together. Then connect one end of the extra bit of conductor to the box using a grounding screw, or a grounding clip.
More recently, guys have been using Greenies from Ideal industries. They're basically a twist-on wire connector with a hole in the top. This method is similar to the long tail method, except that the long tail feeds through the Greenie, and the remaining grounding conductors are connected to the long tail using the Greenie as a twist-on wire connector. The bit of wire protruding from the end of the Greenie is then wrapped around the ground screw, or attached to the box using a ground clamp
Twist-on wire connectors
Greenie® Grounding Connector