Thank you kuldebar, I am glad you found this useful.

aaronfranke, basically I made the mod by making copies of the game files in a mod folder.

1. Located the game files that had the information I wanted to edit:
1a. species (Stellaris\prescripted_countries\99_prescripted_countries.txt)
1b. start screen message (Stellaris\common\start_screen_messages\00_start_screen_messages.txt)
1c. localization files (Stellaris\localisation\english\)

2. I copied those files into a new fold for the mod in Documents\Paradox Interactive\Stellaris\mod\

3. Added my content to the files, removed existing data, and renamed them so they don't overwrite the default files.

4. Lastly added a .mod file in Documents\Paradox Interactive\Stellaris\mod\ to describe the mod (it is a basic text file)

If you take a look at the mod download in <steam>\steamapps\workshop\content\281990\696890457\ (281990 is Stellaris's id on the store, 696890457 is the mod id) you can see the mod's files.

How did you make this mod?

Thank you for this - I was looking for a great human empires mod that I could fill my game with - I am tryingt o mostly play humans with only a few xeno. Really inspired choice to make this mod and fits really well with an imagined back story of my game.