Direct link to instructions so people don’t have to give Reddit webtraffic:
To do this you’ll need:
Your phone
Your Hyperkin SmartBoy
GBA emulator (MyBoy! Is a good one)
Hyperkin SmartBoy companion app
When setting up the app, make sure you do NOT allow it to appear on screen as then you will get a piracy cover/window if you take your phone out.
Go to your emulator and set up the controls (Including L and R).
Make sure that before launching your rom you put phone in your SmartBoy and launch the companion app (as only then SmartBoy will work).
Done!
PS: You can also use it to dump GB games. After the cartridge has been read, go to the folder where it wss dumped and copy it to elsewhere, while doing this, don’t take your phone out! Instead just rotate it.