To make a 2D skybox go to your map and draw 6 brushes (or less if your own map's boundaries block the V O I D, and wrap them around the map, I suggest you don't use a big giant brush then use the hollow tool which just complicates things further, don't complicate something as trivial as the damn sky. Now that you sealed your map from the void you can select the skybox you use, unless you like sky 1 day 1 (the default one), to get a new skybox go to map at the top of your window then properties dont worry that it will select your entire map, now in the skybox texture field click on it then in your browser go to the valve developer wiki sky lists, just search your game in the table of contents then just copy paste the skyname into the said field. Now your skybox works! But now you'll probably want to add a light_environment entity somewhere. This is basically the sun and the outside lighting conditions in one entity. In the same skylists you can see an example map's light_environment information.
Example for ravenholm:
background03 d1_town_01 - d1_town_03 0 12 0 -45 175 230 239 50 43 45 57 5 -1 -1 -1 -1 (.7) -1 -1 -1 -1 (1) 0
Now you got some cool light going on and a skybox! Enjoy!