On the pi, run pip install python-kasa. Change pi in kasa.service to your username. mv or scp the script to ~/ and the kasa.service to /lib/systemd/system/. Use stardard systemctl commands to manage the service, i.e.
# start the service
sudo systemctl start kasa.service
# register the service to start at boot
sudo systemctl enable kasa.service
Register your switches on the Kasa app or python-kasa using the following format.
<GROUP>-<TRIGGER|TARGET>-<DISAMBIGUATOR>
ex: LIVING_ROOM-TRIGGER-WALL_SWITCH
LIVING_ROOM-TARGET-FLOOR_LAMP
This automatically creates a group called LIVING_ROOM and tells the script about the triggers or sources. The disambiguator isn't used by the script, it's for human-readable purposes only. Just make sure that you use - only in the designated places. Any device that you don't want monitored by the script can be called anything, as long as it's not a TRIGGER or TARGET it's ignored.
You can also create Kasa smart routines for the switch as a backup, if desired. Both can be active at the same time.