In this tutorial i will tell you how to make a skybox with Valve Hammer Editor.

I will use the map sdk_water as an example.

*Note* I have highlighted some key areas of the screenshots with black.

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1. Open the map u wish to put a skybox for. [Screenshot A]

2. Then select the Entity's button (the one that looks like a light bulb) and from the drop down menu on the right under "Categories" select "Prefabs" and from the object drop down list select "Box". [Screenshot B]

3. Now go into the middle of your map looking strait down at it and click on the ground. This should make a skybox were you clicked. [Screenshot C]

4. Now resize the skybox to fit around your map. You should make it a tight fit around the walls of your map so that you will not fall out of the map to the bottom of your skybox. [Screenshot D]

5. Don't worry about the green man that came with the skybox nor the random green thing on the floor. They will not make any effect what so ever to your map. [Screenshot E]

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I hope you have learned how to make a very simple skybox from this tutorial.