5. Select the 4 side edges. Mark as seams.
||We want our UV map to have the same shape as the SkyBox texture that we are going to be using.
Cube selected. Edit Mode
Edge select mode.
3D View menu >> Edge Select mode
Ctrl Tab 2
Select four side edges only
3D View menu >> Mesh >> Edges >> Mark Seam
Ctrl E key >> Mark Seam