After around 2 years experience printing on my Geeetech A30, I've come to realise majority of my prints are due to two things. Most predominant issue is the gap from nozzle to bed being wrong, and usually that is caused by the bed being out of alignment.
Auto bed leveling can help here, but a level bed will reduce wear on the machine and also
It's a good idea to do a bed levelling for a new 3D printer or a 3D printer that has recently been moved. It could be an idea to level the bed after a number of prints, usually in the order of 100's hours printing assuming everything is tightened up correctly and the machine does not vibrate too much whitest printing.
Given that we are working with a relatively flat bed, only 4 corners need to be adjusted. Each corner is the same process and the overall process for each corner should be carried out until no adjustments are made. This could mean tweaking every corner 3-4 times. Overall it will probably take 1-2 hours the first time and as little as 10-15 minutes after a bit of experience.
This process is in principle the same for auto levelling beds and manual levelling beds. For non-auto leveling beds, the feeler gauge should be the thickness of the required gap. Auto beds will adjust to the correct gap before printing.
This first pass is to fix any beds that are way out of true and also finding the corner with the smallest gap
Repeat moving the nozzle down and checking each corner until the smallest gap is just slightly more than the feeler gauge.
This process is much of the same process as above, except this time the movements of the nozzle towards the bed are much less. You should be aiming to move down as little as a sheet of paper (0.1mm).
Measuring with a feeler gauge is all about the feel (it's in the name). Every corner should be the same feel.
Continue around the bed taking small steps down until reaching the desired gap between each coner and the nozzle.
The process is finished when you've done a couple rounds to each corner without any adjustment needed.