code:
sky clouds
{
	type=<clouds.tga>;
	layer=1;
	scale_x=1;
	speed_u=8;
	speed_v=16;
	material=mat_sky;
	flags=dome,visible;
}
clouds.tga has to be a seamless 24bit tga texture. you could use a rendering of clouds made in terragen or photoshop, or a photography of clouds as source and then make it tillable...