Be safe — wear appropriate safety equipment (esp. eyes (safety glasses/goggles), and ears (hearing protection — at least foam ear plugs)), ensure clothing, hair and jewelry cannot become caught up in the machine. If necessary, arrange for dust collection and proper ventilation (if necessary, use respiratory gear suitable to the dust particles of the material being milled). Consider the possibility of the spindle starting a fire by friction and take suitable precautions (having a fire extinguisher handy, and other suitable precautions).
Check the machine (all bolts and set screws tight, V-rails in good condition with no nicks or other damage, belts tight and in good shape, wiring in good condition with continuity and securely fastened, and nothing frayed or broken, everything clear and safe). The bolted down, inverted belts which result when using the belt anchor clips make this somewhat difficult — use a mirror to examine the belts while moving the machine along its full range of movement. Note that it is especially important to check the machine after a crash or a failed cut or one which induces chatter or excessive vibration.
Secure the workpiece (right-side up and in the desired orientation) to the worksurface using a technique appropriate to the material (see Workholding). If operating the machine as a 3D printer ensure that the print bed is level, clean and clear of any debris, foreign objects, or previous prints. After securing the workpiece, be certain that the machine is still able to move — this is especially important on the Nomad where the through holes on the table create the possibility of a too-long bolt locking the table to the machine base.
Mount an appropriate spindle and ensure that it is vertical and square to the machine and well-secured.
Examine the End mill to ensure that it is sharp and in good condition and not chipped (this is best done with a loupe or magnifying glass), install it per the spindle manufacturer's directions so that it projects at least as much as the deepest intended cutting depth, check to make certain that everything is clean with no foreign material (which can cause runout or even break endmills), the collet is tight and will not work loose during operation (it needs to be more than hand-tight --- the machine will take no notice or care if it works loose) and the entire assembly is in good condition and spins truly. If making a 3D print ensure filament is loaded and that the spool feeds freely and there is adequate tension to not be overwhelmed by retraction.
Ensure the work area is clear and all cables, wires, etc. run without interference, and that they will not interfere with the machine motion, esp. when homing. Especially check that there is nothing beneath the rails which might interfere with the wheels.
Connect the machine to the computer (power up PC, connect the USB cable, wait until the micro-controller boots up, start the comm/control program, turn on power for the machine (this may just be plugging it in (usually a light will glow on the power supply), some power supplies may have a power switch, or a switch between 110 and 220V, check, once switched on there should be a steady light on the control board which you should be able to see the glow of (other lights (homing switches, communication) will flicker on and off) --- if you cannot see the glow, check the power supply and cable and so forth for switches), open the connection to the machine). If necessary, home the tool to the proper place in relation to the workpiece. (In the absence of limit switches, machine origin will be the location when Grbl powers up.)
If using a Probe be certain to remove the ground clip and secure it safely outside of the machine's working area. Ensure that nothing has been left in the work area. (Optional: Traverse the working boundary of the job as a final check.)
Browse for the NC file which you have already simulated and send it to the machine, following all prompts for tool changes and starting the spindle as required and setting it to the correct speed.
Monitor the machine while it operates, ensuring there is no build-up of dust, debris or fumes, and that nothing works loose, keeping clear of the work area. Do not reach into the machine’s working envelope, nor insert any object into it while the machine is operating. Once the job is complete, turn off the spindle, return the gantry to the home position, or a known offset from home (one may want to wait to turn off the spindle until it’s been homed) and ensure the end mill has stopped spinning before removing the finished piece and any waste. Store endmills carefully when not in use so as to protect the edges. Collets and accessories should be cleaned between uses — wiped off with a suitable solvent such as isopropyl alcohol.