To configure RepRapFirmware for a CNC machine, follow the instructions for configuring RepRapFirmware for a 3D printer using the same architecture (e.g. Cartesian), ignoring anything to do with extruders. You still need to define at least one tool using M563, but it can have zero associated heaters and zero extruder drives.

To put the firmware in CNC mode and tell it which outputs to use for spindle motor control, use the M453 command, see Gcode.

If you are going to use work offsets, by ending the config.g file with a G54 command, those will be used by default. To modify the offsets use G10 L2 P1 or G10 L20 P1 commands.

You will probably want to change the pause.g file so as not to move the tool sideways in case you are edge milling.

Be sure that the stop.g file exists. Otherwise pausing a job will also disable the stepper drivers and the machine would need to be homed again.