You'll need to use a built in app to add/control Kasa devices, start here:
It's pretty straight forward, first time takes a little longer but overall it's pretty quick.
Just try and set up DHCP reservations for your Kasa devices on your router right from the start.
And if you get stuck you can check the original and related, but no longer required since it's now built-in, mega-thread instructions here.
