After leveling:

To ensure your mesh gets used on every print from now on, go into your slicer settings and look for the start GCode.

Look for the Z-homing (either just G28 or G28 Z0) command and insert these two right underneath it:

M501
M420 S1

Your printer should now correctly print first layers even on a warped bed.

When working on the printer, installing a new hotend or nozzle or the bed warping over time, a new Mesh Leveling procedure is recommended.

Manual commands for use with OctoPrint etc.:

G29 S1 - Start MMBL
G29 S2 - Next Mesh point

Raising Z: 
G91 
G1 Z0.02 
G90 
(one after another, not in one line)

Lowering Z: 
G91 
G1 Z-0.02 
G90

After seeing ok in the console, send M500 to save.

Testing your bed leveling

No need to download or create a bed leveling test, simply send those commands to your printer:

G28
G26 C H200 P5 R25 Q4.2 Z4 B65