Skybox Editor
Note:This tutorial was written for 
XBLAH's Modding Tool v1.18.1. Please be aware that subsequent updates might have introduced alterations in terminologies, introduced novel features, or reorganized certain elements within the interface.
Skyboxes can be created straight from image files with the XBLAH's Modding Tool using an intuitive interface, that creates all the required vtfs and vmts. All you have to do is select each cubemap face, preview it, and save. It has a built in 3D previewer to check if the skybox is seamless, and supports the regular square skyboxes and the clamped ones. It will also give you the ideal settings for light_environment, env_sun and env_fog_controller for each sky.
Contents
Accessing the feature
This can be accessed within the tool via 
Top Menu > Materials > Skybox Editor.
Creating a Sky
You can create a new sky through File > New. Load each individual SDR and/or HDR face in their respective Cubemap toolbox. You can preview how the faces align and rotate the skybox using your cursor.
light_environment, env_sun and env_fog_controller settings
The Skybox Editor also provides ideal brightness, ambient, sun and fog configurations, that can be copied to the clipboard already in 
Hammer's property format.
