Hi

More or less on the BL Touch, you need to:

1) Remove the Z limit switch
2) Verify that the BL Touch fires up (light blinks) and works (light switches on when probe is pushed).
3) Design a mount that puts the switch point (not the tip) of the BL below the hot end tip *and* that puts the tip when stowed above the height of the hot end tip.
4) Measure the offsets of the probe tip (X,Y,Z) to the hot end tip. Enter them in the firmware.
5) Turn on bed leveling in Marlin. How you do that varies a lot depending on which release you have.
6) Verify the X,Y,Z offsets and correct the sign errors you may have made
7) Run a bed level and see if the Z crashes (or not), modify the Z offset in firmware to get it close.
8) Step and repeat 7 until you have it close (0.2 mm is very close, 2 mm is not close at all)

Since there are several versions of bed leveling and many releases of Marlin running around, any deep dive into my setup may actually be totally wrong for your setup. On a MakerFarm, reading through Colin's firmware is the best example you have.

Bob