WHAT IS THE MOST EASY WAY TO HOOK UP MILL IN THAT AUTOMATICALLY STARTS WHEN CYCLE STARTS, AND STOPS GCODE THROUGH?

Does your breakout board (interface) have a relay? If so, you can connect one of the router/spindle's ac lines through the relay. The M3, M4 and M5 codes are used to control the router/spindle. Typically, the M codes are automatically added to the g-code regardless if you configure this in your CAM software. If you have the breakout board from buildyourcnc, you can go to the product page for the breakout board and the relay connection diagram and narrative is in the instructions on that page.