How do you install mods?

[solisaureus]

So I downloaded the "Historic 1900 Fleets" Mod, and I'm given the file, but how do I install it? 

[aeson]

Historical Legacy Fleets is essentially a set of slot 3 save states. In order to use it, extract the files and store them somewhere, then copy the files from the appropriate Historical Legacy Fleets subdirectory into the the Game 3 save directory (e.g. if you want to play the 1900 start as Germany with Historical Legacy Fleets, copy the files from the Germany 1900\Game3 folder over to RTW2\Save\Game3), overwriting any existing files in the save folder.

You can also use the files for other save slots, but it's a bit inconvenient as you'll need to change the extension based on which save slot you're putting them into - the extension format is *.[slotnum][nationnum], where [slotnum] is the Arabic numeral corresponding to the save slot (1, 2, 3... for slot 1, slot 2, slot 3...) and [nationnum] is the Arabic numeral corresponding to the internal number for the power which owns the ship. You should only change the [slotnum] when modifying the extension to shift it over to a different save slot.

If you want to do so, you could also use this with an ahistorical legacy fleet for the player-controlled power (either manually or automatically generated), but in order to do so you'd need to create a game, generate the legacy fleet and legacy construction program, save the game and exit, then go edit RTWGame#.bcs so that it contains the information for the player-power that was generated during game creation and the information for the other powers which is found in the appropriate subdirectory of Historical Legacy Fleets, copy over the design files for every power other than the player power, and reload the save, which could be more trouble than it's worth.

[wlbjork]

That is the simplest one.

Extract the file

Open the folders extracted until you find a list of folders with country names on.

Decide which country you wish to play, and open that folder, you will find inside a new folder called "game 3" - copy the "game 3" folder

Now, navigate to your Rule The Waves 2 folder (C:\NWS\Rule The Waves 2 is the default). Open this folder, then open the "save" folder.

Paste the "game 3" folder into this folder. Note that it will over-write any existing saved game in location 3, so be sure you do not want this game before you do.

Load RTW2, and load game 3 - and enjoy!

Note: When updating it is very unlikely that "Historic Fleets" will interfere as it does not change files in the game folder, only the save folder, so don't worry about the warnings about mods.