Stainless Steel is a mod for Medieval II: Total War - Kingdoms, created byechuu. Description: A combination of bug fixes, many small mods, better AI and AI armies, more provinces, graphical improvements and many other campaign and battle map changes.
First, you go over to into the forums for the total war game of your choice and read about them, how to install them, and even where to download them. Be aware, there are two working versions of Medieval Total War and MOST of the mods have not been converted for the Steam Version.
To know the difference it will not be an installer, it will download as a compressed file with a folder inside with maybe install instructions in a text document, and it will have Steam Launch Options as part of the install instructions. Using anything but mods made specifically compatible using Steam Launch Options and you risk breaking your Medieval 2 totally to where deleting and redownloading does not fix the problem.
Some will work and some will not. Some will cause problems the first time you try to run them and some will pick up the problems later causing anything from serious lagging and complete crashes where you cannot load a saved game or a new game and in some cases the game doesnt start at all. Some will work until you try to uninstall to run another mod and then break your Medieval 2. It all depends on what vanilla files get overwritten when it is not foldered and organized properly. Also be aware, older mods not meant for Steam version need to have the vanilla files unpacked and Creative Assembly removed the unpacker and stopped supporting unpacked games. 
It can still be done if you are determined, but know the risks. Originally posted by:First, you go over to into the forums for the total war game of your choice and read about them, how to install them, and even where to download them.
Be aware, there are two working versions of Medieval Total War and MOST of the mods have not been converted for the Steam Version. To know the difference it will not be an installer, it will download as a compressed file with a folder inside with maybe install instructions in a text document, and it will have Steam Launch Options as part of the install instructions. Using anything but mods made specifically compatible using Steam Launch Options and you risk breaking your Medieval 2 totally to where deleting and redownloading does not fix the problem. Some will work and some will not. Some will cause problems the first time you try to run them and some will pick up the problems later causing anything from serious lagging and complete crashes where you cannot load a saved game or a new game and in some cases the game doesnt start at all. Some will work until you try to install to run another mod and then break your Medieval 2. It all depends on what vanilla files get overwritten when it is not foldered and organized properly.
Also be aware, older mods not meant for Steam version need to have the vanilla files unpacked and Creative Assembly removed the unpacker and stopped supporting unpacked games. It can still be done if you are determined, but know the risks. 2 risky to do it, 2 afraid to corrupt game. Originally posted by:1) Get mod files 2) Turn mod files (usually an.exe for the notable ones) into a folder 3) Shove folder in 'mods' folder 4a) Use Inardesco's method 4b) Rename one original expansion folder - say Britannia, and then rename your mod to whatever you just renamed (british_isles), and then tell Steam to launch the Britannia campaign 5) Almost invariably crash on startup Oh, and here's TATW if you want it. HTW is very similar to install as well.
Renaming folders to lay a mod is not much different than playing Russian Roulette with your Medieval 2 as the player. Head on over to the official Total War Support Forums and read the warning not to do that but to use Steam Launch Options. They never really tell you why, but go back reading a few months through the forums here or there and you will see why. Pick any three month section in time since the patch removing Gamespy making the Steam Version no longer the same as the disk version and you will find where it damages the Steam cache for Medieval 2 where even deleting everything and installing clean is even still broken. It is your 30 day wait to be able to install the game again and play after leaving it uninstalled long enough for the cache to manually reset due to time. Now, some older mods with installers do work if you install them to their own location NOT to Medieval 2, take that install folder, put it into the mods folder in the Medieval 2 folder, use Steam Launch Option simply using the formula --features.mod=mods/name_of_the_mod_folder (i.e. --features.mod=mods/RedUnitsMod) and it may still work, but allowing an exe file to overright vanilla files in any way is dangerous!