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.