You can’t tune the driver if it’s not connected to anything. It needs to be in the ramps board, with the power on, but the software running on the Arduino needs to disable the motors. That should be the default state of the motors, or you could send M18. When the motors are disabled, they spin freely. So you can just push the bed around, etc.
When the Arduino enables the motors, it sets the “enable” pin to the driver board, and the driver starts sending current to the motor. If the Arduino has the motors disabled, then the driver can be tuned, and there will be no current going to the motors while you are moving that little screw around.