You can play the Monophonic Ringtone Synthesizer in three ways.
First, you can press the keys on your Qwerty keyboard. For example A, W, S, E play the notes C, C#, D, D# and so on.
Secondly, you can click the notes with your mouse (or tap with your finger).
Thirdly, if you are using Chrome, you can connect a MIDI keyboard, and play via that method. If it doesn't work first time, try refreshing or re-connecting.
If using the Qwerty Keyboard method, here are some example tunes to get you started. Use the comments form below to post examples of your own!