Importing the Avatar Recorder
There are a few ways to import a new package into your Unity project:
To import a custom package, go to Assets > Import Package > Custom Package.

Then find your package using the file explorer

Alternatively, you can also drag-n-drop the package directly into your Unity Project's asset folder.
Find your asset in the file explorer and drag over into Unity's project windew in the Assets folder.Click Import

Go to Unity Asset Store:
Sign in
Find the Avatar Recorder by searching it in the search bar.

Add it to your cart and purchase

Go back to the asset after purchase and click on the button, "Open In Unity".

This should open the package manager in Unity with the asset.

Import it from the Package Manager,
Hit import again and your assets will import in.
Viola, the new Avatar Recorder package is now imported into your Unity Project!