Question: how to install fspy addon?
Answer: Open Blender > Preferences > Add-ons > Install Add-on > Select the downloaded 'FSpy-Blender-1.0.2.zip'
Enable the add-on.
Open the FSpy Windows client and Open a reference image. ( tweak the perspective as necessary)
Save the "tweaked" image as .fspy"
Import the .fspy image and you're good to go.