It's pretty simple. Just create a VERY low polygon version of your model (you can be extremely liberal with how little polygons you use). Then make sure the entire thing is smooth (no edgesplit modifiers). Then export the smd with an appropriate name such as example_physics.smd.

Now in your qc file you will need to specify the collision model:


This is assuming you are compiling the old way and not with the in-game importer. The in-game importer doesn't support collision models.