Overview
This guide will teach you something about making decent planet mods.
You can’t
Planet modding is a selfish clique and the knowledge around it, other than very basic information that was provided years ago by someone from the outside, is considered a “trade secret.”
If you want to make a planet the best you will be able to do is change the topography of a stock planet with an identical atmosphere, with no scatterer or EVE effects, and plop it somewhere in orbit of Kerbol.