custom skybox

ok so I have a custom skybox extracted from my friends map (his HDD crashed) but it is 6 sides, how do i apply this, sorry if this is stupid question.

Skybox files belong under “…\materials\skybox”. Check a .vmt for proper placement.

Map / Map Properties / Skybox Texture Name

The name of the skybox is the .vmt file path starting under “…\materials\skybox” and ending at the directional suffix.

This might help: 2D skybox