Build a Skeleton by adding Joints. The model's vertices and edges remain visible and any Skeleton is drawn superimposed on top of these. The root node may be the only visible node, but a rough Skeleton may be visible due to prior use of the Names Hierarchy Dialog.
Adding a Skeleton Joint is similar to building a curve with the 3D Plot Tool. Click and drag the 3D Cursor near to the node at the end of the Joint to which the new Joint is to be added. Hold down the left mouse button and click the right button to attach the new joint to the one at the 3D cursor. Click and drag the rubber band line until the 3D cursor is at the position for the new node. Hold down the left mouse button and click the right button to create the new joint with it's node at the 3D cursor.
This is all sort of like sticking a pipe-cleaner armature into a clay figure. The basic shape or mesh is there, you are applying the organizational structure for controlled movement.