Solder wires to the modules so they can be programmed.
Connect the modules to the Raspberry Pi.
Setup the programming environment on the Raspberry.
Compile the code and download it to the modules.
Make a case or modify one to accommodate the transmitter.
Extra HW is a push button, a CR2032 battery holder, a resistor and a LED.
Connect the receiver to the car's alarm. Disconnect the alarm's receiver by cutting the PCB trace that connects it to the microcontroller.
Extra HW is a 3.3v regulator (or module) to connect it to the 12v.
Additional features: connect the receiver to the car's board computer to display low battery warnings.