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Dancing_Lobster shows us how to create a custom skybox and enter worldspawn settings for more control in your levels.

Hello,
I'm going to teach you how to make a skybox and how to enter worldspawn settings.
If you have watched Foyleman's video about the basics, you might remember he said something about leak errors. To avoid these errors, you need a skybox. A skybox is a big box around your entire map, with a sky texture.
I made a small test map for this tutorial, it looks like this:



Step1:
Draw out a big brush under your terrain, and make it a little bit bigger than your terrain:



Step2:
Make 4 walls around your terrain. Make sure nothing is sticking in the walls, I allways make the skybox a bit bigger than my map itself. Your walls may overlap each other. Then draw out the 'roof' of your skybox:



Step3: (you can skip this if your walls are not overlapping)
This step is difficult to explain, but I'll do my best.
If your walls are overlapping: select one of you walls and press the 'E' key. Some blue dots will appear. Use your mouse to adjust the angel of the wall, so it doesn't overlap anymore (make sure to use the top view!). Do this with all your walls:



This is what it should look like when you're done:



Step4:
Select the inside of your skybox, by pressing shift+control+LMB. In the textures tab, select textures->usage->sky. Find a nice sky texture (I used mp_strike in this tutorial) and apply it to the inside of your skybox:



That's it, now you have a skybox. Next thing to do are the worldspawn settings. These settings determine the sunlight, the suncolor, the sundirection, etc. Without these settings your map will be very very dark.
First press escape to make sure you have nothing selected. Then press the 'N' key. The entity editor should open, and 'worldspawn' should be selected:



The worldspawn values have to be entered in the "key" and "value" fields. The official settings can be found here.
I'm gonna use the mp_strike settings. So you scroll down until you find the settings you want to use, and copy them. So when you use mp_strike, you typ "contrastGain" in key, and "0.425" in value (all without the quotes!), then you typ diffusefraction" in key, "0.5" in value (again without the quotes), and so on. And that's it!

I added some reflection probes and spawn points in my map, and compiled it. This is what it looks like in-game: