i don't know how to make sun made of two layer and custom sky.

I have only ever really seen things like this done with assemble transformers, for example in Galacticraft. If you would like to see an example, then I believe the Galacticraft source code is open on github. If not, then NTM-CE HBM 1.12.2 and techguns ore open on github and both also mess around the sky and sun. This is the best I can tell you for now, so I'm sorry if it's not very concise or helpful. But if I may ask, what do you mean by "two layer". Isn't the sun a single flat texture in the sky? Do you plan on putting a mask over it, thus making it two layer? Although what you mean by two layer sun is not very clear, I believe it is clear what you mean by custom sky. I would recommend you use gimp to make a TILEABLE sky map. If you already have a sky texture, then you should add it in to your assets folder for your mod as long as whatever else you need for what you want done with the sun.

Again, this is the best advice I can offer for now.

From what I’ve seen, folks usually do a “two‑layer” sun by drawing two textured quads (one with an alpha/mask) and layering them, and for a custom sky a tileable sky map in your assets works best.