In this guide I'm going to show you how to create Episode Two-styled 3D skyboxes. For now, I'm just going to teach you how to create the mountains.

First of all, you'll need a room outside of the map and texture the walls with tools/toolsskybox.
Place a sky_camera entity near the middle of the room, this will be the center of the 3D Skybox which the player can see from the map.

If you have the room with the sky_camera, switch the object type from block to arch at the right bottom of your screen. The categories can still Primitives. Create a "box" with the same size of the room, but don't drag it into the skybox walls. Then press enter, and a window will show up with the settings of the arch. Copy the following:

Wall width: 16
Number of sides: 8, 10, 12 (your choice, im using 8 now.)
Arc: 360 (you can also click the circle button)
Start Angle: 180
Add Height: 0

Press OK and texture all the faces with NoDraw (tools/nodraw)

Now let's texture those walls! First of all you need to the Face Edit Sheet. (Shift+A), then select the faces which are looking to the sky_camera (so the inside faces), then apply a mountain texture on it, click browse and type "mountain" and you'll get a bunch of skybox textures. Maybe the bests are "mountainrange_far_04a, b, c, d" so select one of these and click to "Fit", if it doesn't fits correctly try to change their heights, or play with the texture scale values. If it fits right, select one of the faces and press Alt+Right Click to one of the adjacent faces, then select the adjacent face and repeat it on all. It's not so an important step, but this will make the textures looks correctly.

If you now compile the map, the mountains should appear in the skybox.

PS: Someone says the map has a better optimization if you resize the skybox, now this step is not so recommended, because the skybox can take to close to the actual map.

There might be mistakes with my english grammar, sorry.