If you have A6 6.11.4, you can do this:
Code:
sky cubesky // A code for the sky.
{
type = <cubesky+6.pcx>; // Use the sky bitmap.
flags = cube , // Use a new sky system provided by A6 6.11.4.
visible; // Make it visible.
layer = 1; // Layer for the sky;
z = 0; // Adjust the z position of a cube. X/Y are ignored.
material = mat_sky; // Assign a sky material to a sky.
}
For sky bitmap, make sure you have it in a special order: south, east, north, west, bottom, top, and when saving image, make sure that you have "+6" at the end of the file name before the extension.