Skybox tutorials always seem unnecessarily complicated. I could try to summarize a good way of doing it:
Create a sky_camera at the exact center of the mapping grid (this is Sky Camera A). Alt+enter on it. Go to the “Visgroups” tab, hit “edit groups,” and then hit “new group,” name the group sky_camera_a or something. Go back to the original visgroups tab, and assign sky camera A to the visgroup you just created. You can toggle it on or off in the visgroups tab on the righthand side of hammer’s interface. Create a NEW sky_camera (sky camera b), drag it off to some location outside the map area and hollow out a skybox brush around it. Always make sure Sky camera A is toggled off when you compile the map. Sky camera B should always be in the same spot.
Whenever you want to add to the 3d skybox, simply create new brushes at the edges of your REAL (non 3d skybox) map area, when you’re done making them, select them all, toggle the sky_camera A back on, copy both your brushes and sky_camera A, and copy/paste them somewhere. Make sure all of them are selected (and that includes sky camera a) Go to tools/transform, click “scale” on the left hand side of the window and then type in .0625 for all 3 slots (xyz) and transform it. Drag both the new miniature brushes AND sky camera A together to the position of sky camera B, and line up sky camera B with sky camera A. once they are lined up, delete the sky camera A copy. Now your new miniature brushes will be in their correct place in the 3d skybox.