The exact list of parts needed is ready to order on Reichelt.de: https://www.reichelt.de/my/1454947
The circuit of the synthesizer is based on the Nebulophone by Bleep Labs, you can read the guides
and tips about it here: https://bleeplabs.com/store/nebulophone/
A 9V power supply with hollow plug (positive pole inside) is necessary for the operation.
The output audio signal is monophonic.
Of course, the microcontroller still has to be programmed. The Arduino code is
available at Github.
The easiest way to install it is by using an older Arduino Uno with a capped
Otherwise, there are many tutorials on the net, for example this one: ArduinoToBreadboard
Soldering instructions for beginners: https://www.youtube.com/watch?v=QKbJxytERvg