You say you have a particular galvo (6215H) but don't mention the model of the servo driver board. That is what will determine how you connect to the PC. Most servos are analog IN which means you'll need some type of data acquisition card in your PC to control it. Once a servo is tuned to its galvo/mirror you can just send the system a command signal which will move the mirror in a controlled way. What type of command you send (+/-3V, +/-10V, etc.) again depends on the model of the servo board. If it's a CTI servo then it will be +/-10 Volts).