There's no easy way of learning to mod. It's a long term process. I advise to start learning Blender (or other 3D programm) and get used to frequently used terms in 3D. There's a lot of tutorials for this on YouTube. If you already know how to use Blender, you can check XIV Mods and check their specific tutorials.

If the mod is paid, you can do a private edit to the mod. You can also commission someone to do the upscale, if you need to.

If the mod is free, you can release the upscale.

I don't do upscale commissions. You'll have to find someone that focus on upscaling mods to the body you want.

I open my commissions from time to time. I always announce when I open them on my discord server.

You can also see my information regarding commissions here.