And now for the tricky part. To go through it, you should know how to place the cursor over a particular vertex, and how to set a new origin for any object. Here we go.
Select the eye socket newly created. Copy it (SHIFT+D) and mirror it (S-KEY, X-KEY) and press RETURN. Now, move it somewhere outside of the head and separate it (P-KEY) from the other vertice. Select the socket and enter edit mode (TAB). Select one of the outer vertex and set the new center of the object on it (Center button). Now exit the edit mode (TAB), select the head and enter the edit mode (TAB). Choose the inner vertex corresponding to the one to which you set the center at, on the eye socket. Use SHIFT+S (SNAP Curs->Sel) to set the cursor on it. Exit edit mode and select the eye socket. Use SHIFT+S again (SNAP Sel->Curs). The socket is now perfectly in place! Select the eye socket and then the head. Press CTRL+J to join the meshes. In edit mode (TAB), select all the vertice (A-KEY) and press the Rem Doubles button.