Well, maybe the best move is to contact Canadaman... he is the mod leader from "sins of the prophets" ( https://forums.sinsofasolarempire.com/373320 )...

Reason is very simple... planet are part of a mod... need mesh, texture, entity file, etc ...

Some time ago, i have made a sun/planet mesh who was able to use map at the ratio 2:1 without having unused surface... these new mesh is by example compatible with all planet/sun texture made for Celestia ( by example http://www.celestiamotherlode.net/catalog/fictional.php#1800 )...

As for halo ring, it is a no way now... well, can make a model of it who replace a planet but it will not be functional... have try to make it a neutral thing who can be captured and be used for kill everything in one star group in a multi-star map... but it was only working with human player since AI never used superweapons ( seem that dwith diplomacy thing have change )...

Ark and shield worls is only a mesh thing along with some entity encoding...

Planet need to be part of a mod related to Halo with the races... mainly due of them being together in the galaxydef files... with map being made for these mod... so, it will not be a bad idea to become a members of the Halo mod and work together... i think that they have already someone who have work on map 1 or 2 year ago...

First, you need a modeling software... in my case, i use wings3d who is free

For the texture, you need a 2D software... i use Gimp who is free...

You need XSI mod tool for hardpointing and hook the texture... free software too...

Need the stardock/ironclad utility for convert XSI to .mesh

Need notepad for edit/create the entity files...

That all for the programs that you need !!!

Now, a "step-by-step" or "how" about everything that you need to make is "no"... too much to explain since you don't give any information about your skill and knowledge... do you have some experience in modeling, segmenting, UV editing, texturing, coding, etc...

It is like you ask a "step-by-step" on how build a computer, starting from Silicium... nobody will explain you how to make a mod unless you are already somewhere and need help for somethi,g very specific... not that we are bad people but explain in the detail how to make a mod will take a lot of time maybe more time that making the mod itself...

First read the game documentation about modeling.... or best, read http://soase.weebly.com/ who have tutorial related to modding sins... once you are done with your personal education process, comeback with some more specific question...