How to Import Addons for Dummies
Step 1. Download the addon
I recommend CATS as it has plenty of tools to use for creating avatars for VRChat.
Once you have the addon downloaded, it should typically be in a .zip file.
Do not unzip/extract the file or it may not work!
Step 2. Open Add-ons
In Blender, at the very top left of the window click "Edit" then "Preferences" click "Add-ons" in the new window.
Step 3. Actually Import and Enable It
Click "Import" in the top right of the window next to "Refresh" and select the addon .zip you downloaded. It may take a moment.
Once you have imported the add-on click the searchbar in the window and search the name of the addon, then click on the checkbox to enable it. You may have to restart Blender in some cases to ensure it works properly. Please remember to read the documentation of the add-ons to find out how to use it.