Install Tone Library
Download and install the Tone library from https://code.google.com/archive/p/rogue-code/wikis/ToneLibraryDocumentation.wiki
This library is abandoned and hasn't been upgraded for the Leonardo board, which is basically the LittleBits board, so you need to download and replace Tone.cpp with the attachment in the third to last message from this page: https://github.com/gugiar/rogue-code/issues/13
Once you do that you should be able to play 2 simultaneous tones. If you want you can play 3, but that will take over the timer also used for delay() and millis() functions: if you try to use those functions while playing 3 notes your code will hang.
Build the circuit
Following the picture, on the left side of the arduino bit hook up a dimmer, or other variable source, and a button, with power going to each of them. (BitSnap, splitter, whatever.)
On the right side, the top pin goes to the trigger pin of an envelope, the middle pin is the bass line, which goes directly to a mix bit, and the bottom (melody) pin goes to the input of the envelope.
The output of the envelope goes to the second mix input.
Hook the mix up to a filter to help with the popping you get from arduino synth, and snap a synth speaker onto the end.
Load code onto the arduino bit
Copy the code from this page and paste it into a new sketch.
Use the arduino cable to upload auto_piano.ino, and then tune envelope, mix and filter knobs to taste.
Write your own music
Right now my code is set up to play 'This Pretty Planet'. To change it to anything you like, just change the notes declared in melody and the chords declared in bass, the notes in melody should match up 1-to-1 with the bass chords. The code will auto detect the length of the array, but they need to be the same.
You can also change which notes are defined in notes (which will of course change which numbers you use in melody, since melody is just an index into notes, and you can also change the chords, which are set up in chords using any of the notes defined in Tone.h.