You're welcome gtsgames!
In the assets pack, check out the tutorials folder, for more tips on the methods I use.
To make the skyboxes, I use 2 free online tools tools..
1- https://www.manyworlds.run/
2- https://matheowis.github.io/HDRI-to-CubeMap/
Open manyworlds and add a text description for the skybox you want. (eg: post-apocalyptic concrete wasteland covered in ivy and grass)
Then save the HDRI map it generates...
Next, simply upload that HDRI map to the cubemap generator, select the output quality to convert it into a skycube.
I'm using 256 pixels as an output setting for my skyboxes in but set it to whatever you like. (usually either: 16, 32, 64, 128, 256, 512, 1024 pixels, which is the resolution for each side of the cube).
I spend an hour or so each time (it's quite good fun) generating some skyboxes, then test them in a scene to see how they look. I keep the best ones and delete the useless ones. If the horizon sometimes looks too high or too low in a test scene, I'll edit the HDRI manually (using a basic graphics app) to shift the horizon up/down, then feed it back into skybox generator again and test the new version. I guess I could try adding "central horizon" in the description to see if that helps.
As a tip using manyworlds: if you stumble on a good result, you can use that same text description again to make multiple scenes, by adding a random seed/string to the end of the description.... eg:
"post-apocalyptic concrete wasteland covered in ivy and grass 6784r5t6"
"post-apocalyptic concrete wasteland covered in ivy and grass dcft5t6yf"