You would add an entry to the GC2Types.xml file, at the end but before the final /GC2 tag.
It would look like this:
[Hull Name="Romulan Warbird"]
[DisplayName]Warbird[/DisplayName]
[Description]The most powerful ship of the Romulans[/Description]
[Cost]160[/Cost]
[Size]80[/Size]
[SensorRange]12[/SensorRange]
[HP]70[/HP]
[Model]Warbird[/Model]
[Thumbnail]Warbird[/Thumbnail]
[Tech_Requirement]Massive Scale Building[/Tech_Requirement]
[Speed]25[/Speed]
[Category]Huge[/Category]
[Logistics]8[/Logistics]
[/Hull]
To paste in in to the browser the brackets have been changed from < and > to [ and ], you will see what I mean when you open the GC2Types.xml file. (The replace function in Notepad is good for changing them back easily)
You can open and edit the file with Notepad or Wordpad, or any number of XML editors.
Back up your original before you modify it.
In the above example I am using the Romulan Warbird model. The .x model file (this file would get a S99_ prefix to make it available to all races) and associated .png image files go into the C:\Program Files\Stardock\TotalGaming\GalCiv2\Gfx\Models folder (assuming you used the default install location).
This ship was given an increase in its abilities, as you can see. The sensor range was increased to 12 (which is the max), the speed increased to 25 (add a few more engines and you can get 45-60 movement points easily, and still have room for plenty of weapons/defenses), and the hit points were increased.
Any changes will not show up in a saved game, only in new games. Then it will show up in the Huge category once you have researched Massive Scale Building.