Having established the proportions and basic structure, the rest of the work amounts to adding such additional rows and columns of vertices (using the Editable Patch subdivide tool) as are necessary to support the desired degree of detail.

I've kept this character as simple as possible for instructional purposes, but this structure would support a very considerable range of possibilities.

If subdivision surface modeling is to provide the advantages of spline modeling, the grid structure must be absolutely regular and clean. The rows and columns of edges (or vertices) must be easily identifiable and selectable. If you can read the structure of a mesh easily, you can edit it.

When rows and columns of edges are horizontal or vertical, they are easy to select and scale to adjust the shape of the limbs. The arm presents a problem in this regard. I like to model the arm slightly downward from the sides, instead of straight out, because this is a better neutral pose for deformation during animation. This means sometimes having to rotate the arm while modeling to make horizontal, and rotating it back into the proper pose after scaling columns of edges.