What do you need?
You need a guitar and a way for the application to hear you. I recommend plugging an electric guitar into your PC via an audio interface. You can also use an acoustic guitar with a phone or laptop. In that case, make sure you're in a quiet environment so the microphone won't pick up background noise. You might also need to disable the sound effects in game settings.
Important note
Once a note is successfully recognized, you won't be able to proceed until you mute the strings. This prevents the next note from being mistakenly recognized if it's the same, and it also makes it easier to recognize the next note when no other strings are ringing.
Sometimes you may notice that you played a different fret or string than the one shown, but the note was still recognized as correct. This happens because you played the same note in a different position. Here's an example:
Let's say you've set the game to use the first 11 frets (the head part of the guitar), and you're supposed to play a C# on the fifth string (4th fret). You could "cheat" by playing the same note on the sixth string (9th fret). These are identical notes — same pitch, same frequency. However, since the purpose of the game is to help you learn the fretboard, it's best not to cheat!