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.

Tools Required

• Feeler gauge (or any thin item, paper, thin card)
• Perhaps tools depending on model, but usually it is a thumb screw adjuster

Some Assumptions

• The bed is perfectly flat (if not go buy a new bed)
• Bed and nozzle temperature are constant, so either on or room temperature but will remain the same throughout.
• This article talks about moving the nozzle, if your printer has a static nozzle move the bed. If one axis moves on the bed and one moves on the nozzle, it will be a bit of each.
• Regardless of what is chosen to be the "feeler gauge", they all work the same. 

Process

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.

Rough adjustment

This first pass is to fix any beds that are way out of true and also finding the corner with the smallest gap

• Move the nozzle to within a few mm of the bed on any one corner. Around 5-10mm between the nozzle and the bed. 
• Move the noel to all other nozzles watching for the smallest gap.
• If difficult to tell which corner has the smallest gap, move the nozzle down a few mm again and check each corner again.

Repeat moving the nozzle down and checking each corner until the smallest gap is just slightly more than the feeler gauge. 

Adjustment

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.

Finnishing up

The process is finished when you've done a couple rounds to each corner without any adjustment needed.