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