So if you assume "mod" means create a ship (starbases are ships with a few different attributes), why don't you search for:
"creating ships"
"how to create a ship"
"how to create ships"
"adding ships to the game"
etc, etc, etc....?
But if what you want is just adding a mod (of any kind) to your BC, after downloading it, than pretty much you gotta do what moed said.
However tho I would recommend the mod KM1 (which is a pack of a lot of good mods), checking the *Mod List* sticky there is somewhere around here (which contains a list of great mods that work together, tho like I said, KM1 contains nearly all of them), or searching for BridgeCommander Files for any mod you want in particular.