Instructions:

From the home screen, go to Settings > Accessibility > Manage accessibility features.

Under Keyboard and text input, turn on Switch Access. Then you will actually see a bunch of blue highlights around your screen.

A window will pop up asking you to pair a switch. Follow the on-screen instructions if you have a Bluetooth switch to pair.

You can customize Switch Access under Switch Access options. Here you can assign keyboard keys, adjust scanning speed, and more.

You can also enable or disable Switch Access from the Accessibility section of the control panel.