This is an old example, outdated after 1.8 changed how starting systems are set up. But it still serves as a general example of how to go about making your own version of a mod.

- In C:\Users\ <INSERT_USERNAME> \Documents\Paradox Interactive\Stellaris\mod\ <INSERT_MOD_NAME> \common\solar_system_initializers, you should find a .txt file. Open it in Notepad++. NOT WINDOWS NOTEPAD. Notepad likes the wrong encoding scheme and can cause perfectly good code to trigger much fail.

- Replace all instances of building_power_plant_1 with building_primitive_power_plant , building_mining_network_1 with building_primitive_factory, building_hydroponics_farm_1 with building_primitive_farm, and building_capital_1 with building_colony_shelter.

- Save your changes.Now when you launch Stellaris, it should use the local copy of MOD_NAME along with this one, and you'll start with primitive Iron Fisted Lumpaloos or whatever.

- If it works and you want to share it, follow one of the several tutorials about setting up a mod. Name it something like Primitive Iron Fisted Lumpaloos Patch. Copy your modded local system initializer file into it (preserving the full folder path, i.e. \primitive_ifl_patch\common\solar_system_initializers\Iron_Fisted_Lumpaloo_initializer.txt). Now go back to the modding tutorial and follow its advice about uploading.

- To play, activate all three of this mod, the original race mod (Iron Fisted Lumpaloos), and your new patch mod. If it doesn't work, your modded initializer filename may need to come ascibetically before the initializer you're trying to replace. Or maybe after. It's not like Paradox gave us a reference book on this stuff.

- If it does all work, drop a comment here and I'll be more than happy to include your new mod in the Primitive Players mod collection. I want this to be a community effort. If I can mod, you guys definitely can.

Modified the special Sol systems added in Lion's 'Sol System Expanded' mod.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=919932376

...and then modified it so significantly that it is now the largest Sol System overhall mod.

Noice. Thanks much for doing this. If I need to change anything in the initializers that means this needs an update I'll post here and on your mod thread.

Per your request, the size 24 Earth versions of the Sol System Expanded mod have been added in my previous mod. :)
Also worked out the compatability issues that you'd suggested.

If you could drop a quick note on how to make a mod like this compatible I'd appreciate it, I don't think I did a good job above.

Sure thing. I'm not sure if this is any better, but here goes:

(Having your mod overwrite the original)
Pode's suggestion above is to title your modified file's name so that it preceeds the original file's name asciibetically. Asciibetically means alphabetically, but with capital letters coming before lowercase ones. If the original file's name is lowercase, all you have to do is have your modified file have the exact same name, but with a capitalized version first letter instead. If the first letter was already capitalized, capitalize the second - and so on.

Super easy.

(Having your mod run alongside the original)
This is only useful if you want to use the original mod and your own mod at the same time, or don't want to change your mod loadout for whatever reason, or... something.
If you follow this method, the original mod will not be overwritten if both the original and your patched version are loaded at the same time.
-As above, change your modified files to something new.
Example[s71.photobucket.com]
(In my case, I added "prim_" as a prefix to the existing file names.)
Do this with all files in your modified version of the mod to differentiate them for the original. This includes all of the files in the common/solar_system_initializer directory, as well as those in the localisation and localisation_synched directories.
(In my case, again, I just addded "prim_" as a prefix to the existing file names.)
-After this, you need to edit one of the first lines of the initializer. (The file in the common/solar_system_initializer directory.)
Example[s71.photobucket.com]
Look for where the file uses the word "initializer". It'll be right at the top.
(In my case, it originally read "sol_system_expanded_initializer = {". I added in "prim_" as a prefix to this, again. The new version reads "primsol_system_expanded_initializer = {"
-After this, you need to edit the file(s) in the localisation directory.
Example[s71.photobucket.com]
This is where the original mod author probably typed up a description for the special home system. You'll see those notes off to the right, contained by quotation marks. Change this description to include something to indicate that this is the Primitive Players version of the home system. In my case, I literally just added the word "Primitive".
After you've modified the description(s), you'll need to change the text referencing the initializer (ex: sol_system_expanded_initializer) to what you changed the initializer file name to (ex: prim_sol_system_expanded_initializer)
-Now just upload it as per usual, and you should be able to load both the original mod and your modified mod (modmod?) at the same time without worry of conflict.

Also super easy, just takes a little longer - but totally removes any risk of compatibility issues or accidentally loading/unloading the wrong file.

Excellent, thank you. I'll be referring to it myself

1.8 has made much of this moot. The placement of buildings on homeworlds is now handled separately from defining the home system, so various species mods *should* be compatible with my mod going forward.