You need a proper minecraft sky mapping template, Cube the sphere-free is a really good software (but you will still need to edit the output), editing software (Photoshop, Corel, Substance Alchemist, Designer or Painter -all paid or Gimp, Paint.net or just paint-free), read and remember Optifine/MCpatcher guidlines (this is if you really want to learn), finally be patient and creative. It is very time consuming, and yet fun. In order to enjoy it to the fullest, sky mappings should be as close as real life. Lower quality is still fun... but the higher the resolution, the more immersive the experience will be.

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As a side note, check the Downloads issue tab above. It explains where to click ... ONLY CLICK THERE, EYES ON THE TOP RIGHT ... hahaha-just kidding (but it is true, just like in Curseforge), so you can get bigger high def packs. Have fun. I will work on the MCpatcher version later.