Remove the old workings from an old ZX Spectrum. (but never destroy a working one!)
Solder the keyboard connectors to the Arduino Pro Mini via some wires.
Build the USB adapter using the 5 components. A copy of the circuit diagram is in the GitHub repository.
Attach the USB adapter to the Arduino Pro Mini and the USB lead.
Use an FDTI adapter or Arduino Lilypad programmer to programme the Arduino Pro Mini. The source is on the GitHub repository.
Route the wires in the keyboard, stick down the ends of the connectors to stop them wondering, and fix the end of the USB cable so it will not pull out.
Test it with a PC and put the screws in if it works.
Load up a Spectrum emulator, and relive your youth. Then use it to send me an email to and let me know how you got on.