Firstly, make sure the X axis is horizontal, test it with a ruler. If not, turn off the stepper motor in settings, adjusting the coupling by hand till both the left and the right side are at the same height.
Secondly, click "auto home", to let the X Y Z axis go to the original position, and click "level" to start the leveling, put the leveling card or an A4 paper between the nozzle and the bed, twist the bolt at the bottom of the heating bed slightly, stop when the gap is as thick as the card or the A4 paper, level all the corners successively.
Thirdly, all procedures above have done, but the nozzle is a little far from the bed, raise up the Z axis to 10 mm, moving the limit switch downward a little. And level again.
Fourthly, level while the printer is printing, watch carefully, adjust the bed until the first layer is perfect .
Here is the video link: https://www.youtube.com/watch?v=PcViHrhyb1w