Making a collision model for Valve Source in 3ds Max is fairly simple once you are familiar with the requirements. The video tutorial here demonstrates the basics of making a collision model.

A basic list of requirements for making a collision model follows:

1. The collision model should be a single mesh.
2. Each element in the mesh should be a single convex piece of geometry.
3. Although the elements should match the general shape of the full model, the polygon count should be kept very low.
4. Each element needs to have a unique smoothing group applied to it.
5. The whole model needs to have a material that has a Bitmap in the Diffuse map (even though the collision model is never rendered).

The video failed to mention that each element should be convex.

Also see Convexity.