You can download mods and enable them using the Mod Manager. To access the Mod Manager, select the Mod Manager tab on the pre-game Options window (this is the window that appears when you launch the game).
You'll find full instructions on using mods below.
The vast majority of mods are made by amateur developers: many mods have not been thoroughly tested, and some may cause crashes or other stability issues.
Just in case anyone is curious, this does not appear to work on the Mac App Store version of EU/EW. If you root around in the app you can find the files and directories Anderkent talks about and I followed his procedure, but when I try to launch XCom it crashes immediately.
Many mods are incompatible with each other. Enabling incompatible mods may cause the game to crash. If you plan to enable multiple mods at once, we recommend checking the mods' documentation for information about their compatibility with other mods.
It is possible to add mods to a saved game: you can enable mods and load a saved game made before those mods were enabled. However, we recommend enabling any mods you want to use before starting a new game, rather than adding them to an existing game.
It is not possible to remove mods from a saved game: if you save a game while using one or more mods, you must always have those mods enabled while playing that saved game. If you disable any of those mods, you will be unable to load the saved game.
Once you have downloaded a mod, you must install it by following the steps below:
From the pre-game Options window, select the Support tab.
Click on the Open Preferences Folder button.
The “XCOM 2” folder will open. Navigate to VFS/Local/Mods.
Copy the folder of the mod you want to install into the “Mods” folder.
The next time you launch the game, the mod will appear in the Mod Manager on the pre-game Options window.
To uninstall a mod that you've installed manually, you must delete its folder from the “mods” folder. Once you have deleted a mod, it will no longer appear in the Mod Manager.
Once you have installed a mod, you must enable it using the Mod Manager before it will appear in the game.
To enable a mod:
From the pre-game Options window, select the Mod Manager tab.
The name of the mod will appear on an icon in the Mod Manager. Click on the red Off button next to the name of the mod to switch it to On.
The next time you launch the game, the mod will be enabled.
To disable a mod:
To disable a mod, open the Mod Manager and click on the green On button next to the name of the mod to switch it to Off.