The procedure is generally identical to any other Bluetooth device. Below we are showing typical examples.
A. Manually pairing the controller
1. Open Bluetooth manager on your Android device and click "Pair new device". The Bluetooth (connections) manager can be accessed in the System Settings menu - Connected devices (example for Adventure Control /AdvCtrl):
The Android device starts searching for Bluetooth devices in range available for pairing. Press any of the controller buttons and the controller will appear on the list in a few seconds. Click it and confirm the pairing request. If the controller doesn't appear on the list within 30 seconds, please refer to this support article (the controller is either not powered or already connected to other Android device).
On some devices, such as Samsung cellphones, the Bluetooth manager and pairing menu is hiding under additional sub-menus:
Or by long-pressing Bluetooth icon on the pull-down quick menu: 
On some devices, such as Samsung cell phones, is the actual list again hiding under a sub-menu, in our example called "Details":
B. How to manually Forget / Unpair Controller
Open your Android device Bluetooth manager (same proceedure like detailed above) and click the cog wheel next to the controller you wish to unpair/forget - select "Forget" or "Unpair" (it is called differently depending on Android version)
On other versions of Android, such as Samsung cell phones, the dialog might look like this: 
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