Download Installer
Install Mods
Change Directory
Go to https://fabricmc.net/use/ and download the JAR installer or the exe installer.
download Windows installer
Close the minecraft launcher, it shouldn't be open while you're installing the fabric launcher.
close minecraft launcher
Run the fabric installer by double clicking it.
Go to the client tab and select your minecraft version. You can leave the loader version, the most recent one works in any minecraft version. Leave Create profile checked and click Install
install fabric loader
After this, you can close the dialog box and the installer.
close fabric installer
Run your new fabric installation if you don't want to change the game directory, otherwise go to Change Directory.
After running your fabric installation, you should see the fabric watermark in the bottom left corner like this:
fabric watermark in bottom left corner
For now, close the game, so you can install some mods. If you haven't changed the game directory, the mods folder will be in your appdata folder.
To go to the appdata folder: Press 
Win + R, type %appdata% and hit ok.
run %appdata%
After this go to your .minecraft folder and the mods folder.
go to minecraft folder
go to mods folder
Drag your mods to the mods folder (some mods may require the Fabric API or other mods, you can just drag these in your mods folder as well) check if your mods are for the version you've installed, otherwise they might not work.
drag mods in folder
After this close the mods folder and start your game.
Enjoy your newly installed mod!
If you don't want to use the default game directory, you can change it.
You can create a game directory in any folder, so choose whatever you want.
Create a game directory folder in the folder of your choosing and open the minecraft launcher.
In the minecraft launcher, click on Installations and click on the ... next to the fabric installation and click on Edit.
go to installation
go to edit
Change the game directory to go to the game directory you've created and click Save.
edit directory
You can now go back to here.