I have researched and all threads and tutorials seem to be for 1.12 and before. Thanks.
You're going to need optifine to do so. A
I think the way they're done hasn't changed since 1.12, so just research a bit
From 1.13. onwards the "mcpatcher"-folder is no longer recognised and its content would have to be put in an "optifine"-folder instead. File structure and names didn't change afaik.
Also here is the relevant entry for custom skies in the Optifine documentary.