First select version
Write down the exact version of Minecraft for which you are downloading the mod. Check the mod page for Fabric loader, Java edition and dependencies. Even neighboring versions of the game may be incompatible. The Fabric API is needed by many mods, but the Loader and the API serve different purposes. Before making changes, save a copy of the world. For the first test, use a separate world: it’s easier to see what the mod adds and not mix the result with the previous settings.
Installation in the official Launcher
Download the Windows installer from fabricmc.net. Close the game and Minecraft Launcher, select the desired version in the installer and leave Create Profile enabled, then install Loader. The Windows version of the .exe uses the Java runtime bundled with the official Minecraft Launcher. Open Launcher and select the Fabric profile. Fabric developer instructions apply specifically to the official Launcher; another program needs its own instructions.
Where to put the mod
For a standard Windows installation, the mods folder is %appdata%\.minecraft\mods. Place the appropriate .jar files there, including the dependencies listed by the author. If the profile has its own game folder, use its mods: do not select a directory just out of habit. Do not extract the .jar as a normal installation archive. Run the selected Fabric profile, not the regular game profile.
If the game doesn't start
Compare the versions of the game, Loader and each mod with the requirements of the author. Write down the error text and the list of files you added. Check out a clean Fabric profile in a separate folder, then add mods and necessary dependencies one set at a time. This test shows after what change the failure occurred. Save the original files and a copy of the world; re-installing the entire game before finding out the cause may only complicate the comparison. Fabric is a third party project and not an official Mojang module.
Sources and checks
Guides are automatically translated from Russian; interface labels may differ. The Russian edition is the original.