1. M502 to revert to defaults (and deactivate bed leveling)
2. M500 save defaults
3. M503 to print out settings. Notice the M851 Z probe offset line. Whatever you can read here is what is set as probe offset in the configuration.h of the Marlin firmware.
4. M851 Z0 will reset this to 0 for the following calibration
5. G28 to let it home in all axis. The nozzle will be above the bed by the distance of the probe trigger point to the nozzle, plus the safety margin configured in the firmware to move down after homing
6. M211 S0 deactivate software endstops, so we can go past Z0.0
7. G0 Z0 to go to the probe trigger point (in case there was a safety margin)
8. M119 reports the endstops/probe status. Ensure that it reads TRIGGERED for the z-probe. Move up and down with G0 Z0.05 or G0 Z-0.05 to check that you are in deed right at the trigger point where it switches back and forth between TRIGGERED and open
9. M114 note down the exact z height
10. Use G0 Z-x.xx to slowly approach the perfect nozzle height with the paper method. See notes to temperature below.
11. M114 Note down the final height, subtract the initial height of the trigger point.
12. M851 Z-x.yz Set the calculated z offset (negative, if nozzle is above trigger point)
13. M500 Save the value
14. G29 start bed leveling (details may vary depending on your bed level algorithm)
15. M500 save bed level data
16. M211 S1 re-enable software endstops