Its fairly simple, you need to play around in XML files, the main files can be found in your directory:
\SteamLibrary\steamapps\common\Sid Meier's Civilization Beyond Earth\assets\Gameplay\XML\Civilizations
Within this you will find several different XML files but the two main ones you need are: CivBECivilizations.xml and CivBETraits.xml, these files will show all of the information that can be included, how to include it, under what table and which data set should be used (i.e. text, integer, boolean (1/0/true/false)) and may even provide some examples (alternatively you could use another civilisation created buy someone else and modify it to your purposes).
Also look for text files that include, but may all be included into one file (I personally like to separate files to make it easy for adjustments):
Cities Names - Lists the names of all the cities used.
Leader Dialogue - Sets out the text responses to civilisation interactions.
Spies Names - Names of all of the spies used.
Text - Random bits of descriptions, help, strategy and civopedia texts.
Other files also include:
Diplomacy Responses - Sets out the framework for leader dialogue texts
Leader - Sets out the leader and the flavor or bias used by the AI or automated features.
Personalities - I just use default personality
Player Colour - Sets out the colours used buy the player for this sponsor / civilisation.
Just load up modbuddy and away you go, I also use gimp (with a plugin) to create or alter any .dds image files needed but that is an option as existing icons can be used (however this can get confusing).

Hope this helps
