I'll just get you started with the basics and let you play around making more advanced skies later.  For now, just click on the Generate Terrain button.  Click on Close when the terrain has been generated.  Next thing you'll need is a sky.  Go to View | Cloudscape... then click on Add Layer...  They may look funny, but I'd just leave the default settings. You can mess around with it more later.

We need to set up our sky for rendering now.  Go to View | Render Control...  We will need to set the camera up correctly so the sky will fit together seamlessly.  Click on Camera Settings... and set both Exposure and Zoom to 1.  Close the window.  Next we'll need to set the gamma down so the sky doesn't look really bright in Quake2.  On the Rendering Control window, click on Settings... and set the gamma correction to 1.  Close the window.  We need to set the image size now.  In the Rendering Control window, set the Width and Height to 512.  Since we want our sky to look decent, put the Detail bar all the way to the right.

We should be all set to render now.  What we are doing is making a 6 textures for the inside of a cube - aka "sky box."  Set both x and y to 128 in the Camera Position.  Set x to 256 and y to 128 and uncheck Fixed Height Above Terrain in Target Position.  Set the camera orientation to 90|0|0.  Click on Render Image.  This can take anywhere from a few seconds to an hour, depending on speed of your machine.  It usually takes me 10-30 mins.

Once the image is rendered, click on the Save button.  Save it as something_rt.bmp (something = whatever you want).  Only 5 more views to do :)  Render images with the orientations 0|0|0, 270|0|0, 180|0|0, 0|90|0, and 0|-90|0.  Save them as something_ft.bmp, something_lf.bmp, something_bk.bmp, something_up.bmp, and something_dn.bmp respectively.

Now that we've rendered all 6 views, we need to put them in the correct format.  You'll need a program that can save in .TGA and .PCX format.  I personally use Paint Shop Pro 5.  Open all the images and resize them to 256x256 pixels (the reason I told you to render them in 512x512 is because you will get a little higher quality rendering) and mirror them (flip them horizontally).  Save them as \quake2\baseq2\env\something_rt.tga, etc.  If the \env\ directory doesn't exist, create it.  If you are using Lview, you need to flip the image vertically as well (I don't know why, but they are upside-down in quake2 if you use Lview).

You will need to create some .PCX files for all those poor people out there still using software :)  Load the palette for Quake2 or for whatever game you are making the skies for.  You can grab JASC palette files here if you don't have them.  Once you have loaded the 256 color palette, save the images as \quake2\baseq2\env\something_rt.pcx, etc.

You should be set!  All that's left to do is test.  I've made a map that's entirely sky for testing purposes.  You can grab it here.  Unzip it to \quake2\baseq2\maps\, load up Quake2, bring down the console, and type map skybox.  When it's loaded, bring down the console (if it's not already down) and type sky something_ (where something is whatever you named your sky).  It is likely that some of the sections of the sky will be flipped or rotated the wrong way and you'll have to load the images back up and adjust them.

Good luck on making some cool skies! :)  If I left out any details, screwed something up, etc, email me: jitspoe