• Right click on the game in your library
  
• Click Properties
  
• Click Set launch properties...
  
• In the input window type cheat

Or if you want a desktop shortcut:

• Right click on the game in your library
  
• Click Properties
  
• Go to Local files tab
  
• Click Browse local files
  
• Right click on the GalCiv.exe file -> Send to -> Desktop (create shortcut)
  
• Right click on the newly created shortcut -> Properties
  
• Replace the whole Target input with, for example: "C:\Program Files (x86)\Steam\Steam.exe" -applaunch 226860 -cheat

It is not possible to add arguments on shortcuts that Steam creates.

After launching the game with the 'cheat' argument, start or load a game. Open the console with the tilde(~) key*, the key can be changed in input options. If the console has opened, type help to see a full list of the commands. For information on specific commands type help <command>.

* Note that you can't open the console in the Main Menu.

For example writing help modcredits displays:

SYNOPSIS
modcredits <amount>
DESCRIPTION
The modcredits command will modify the local player's credits by the value provided.
PARAMETERS
<amount> - numeric value to modify the credits by.

For an expanded list of all the commands see the GalCiv 3 gamepedia wiki page:
http://galciv3.gamepedia.com/Console_commands