Overview
This is a guide on how to create a SkyBox for your maps.
What is a SkyBox?
The SkyBox is a method of adding a continuous sky above a map. It can add a lot of atmosphere to a map, and make it prettier to look at. You can add a sunny day, a starry night, or anything in between.
NOTE: For the best experience, select Realtime Preview (either by clicking the button on the viewport 
ALSO NOTE: I’m assuming you already know the basics of the POSTed tool, so I will not be going into extreme detail. If you want more info about the POSTed check out the links to various other workshop guides in the closing.
FINAL NOTE (I swear): Make sure you set the viewport so you can actually see the Sky Box. Do this by selecting the Show Sky Box button at the top of the dynamic viewport.
How to add a Sky Box to your map (Basic steps)
Step 1: Subtract a Cube 
NOTE: Here is a reference you can use to find a good size for the cube: Say you’re using a texture set consisting of 8 panels and each is 512×512. A skybox that is 1024x1024x512 high should suffice. A typical skybox is 128*1024*1024.
ALSO NOTE: If your texture doesn’t fit perfectly, you can always right click on the surfaces, then click on Surface Properties, go to Pan/Rot/Scale, and tinker with the Scaling.
Step 2: Go back to the main map and right click on the surfaces that you want the SkyBox to appear on. After this, click on Surface Properties. In the properties, under Flags check Fake Backdrop and Unlit. These make for the optimal viewing experience for your skybox.
Step 3: In the resources browser, search for the actor class SkyZoneInfo (it’s under the ZoneInfo category, Actor >> Info >> ZoneInfo >> SkyZoneInfo), then select it. After you select it, right click in the cube you made and click Add SkyZoneInfo Here. Then just position the actor in the middle of your SkyBox.
Step 4: Rebuild the map (go to Build at the menu bar, then click Build All). Then you’re set! Hopefully if you had real time preview on you will be able to see the SkyBox around your map.
Extra Steps (Not required)
Here are some extra steps you could do to make your Sky Box even more pretty looking!
You can add a Static Mesh to your Sky Box to make it look like the sun is setting.
Step 1: Go to the resources browser and go to to the Static Meshes tab.
Step 2: Go to the Zo_Highland_Meshes category and select zo_skybox_haze. Then right click it and set it to Unlit. To do it with the static mesh right click on the mesh and then go to Static Mesh Properties > Display > bUnlit and then set it to True.
NOTE: If you can’t find the Static Mesh in the browser, click File, Open, then go to the Postal2Editor directory in your Steam folder and go to the Static Meshes folder then open up the zo_highland package. Also make sure you click the All button so you can see all the static meshes in the package.
Step 3: Add the SkyBoxInfo actor in the middle of the Static Mesh. Then you’re done! You added a cloudy dusk/sunset look to your Sky Box.
You can add some lighting that makes it look like the sun is shining on your map! While this doesn’t necessarily have anything to do with the Sky Box, you can use it to add some extra sky effects to your map.
NOTE: You add this into the main part of the map, not where the Sky Box is.
Step 1: Click on the Add Special Actor button on the menu.
Step 2: Go to Lighting and click on Sunlight.
Step 3: Position the sunlight wherever you want on your map, then right click on it and go to Sunlight Properties. In the properties menu, go to Lighting and you can set the lighting radius and other settings to your likings. After you position it where you want, you’re done!
Closing
Well done! You have successfully added a Sky Box to your map. Now, whenever someone plays your maps, they will forget what they’re doing and just stare at the sun. Or however you people play video games.
You can use the real time preview mode in your dynamic viewport to get a good feeling of how your Sky Box will look ingame, or you can just load the map up.
Huge thanks to the Beyond Unreal wiki and their tutorial on how to build a Sky Box, of which I edited to make it a bit simpler to use, and added some extra goodies.
– [link]