Bed leveling is without a doubt the first real challenge most users will face. It can affect everything from print quality to bed adhesion, and your first layer can tell you a lot about what will happen latter in the print.
The ideas as a whole is to get the nozzle so that it is close enough to the bed to leave filament attached to the bed, but far enough away that the nozzle is not smearing the filament, or preventing the filament from coming out of the nozzle.
But first we need to discuss the springs and knobs used to level the print bed. Most 3D printers that use knobs for leveling consist of a heat bed and a bed frame beneath it, that the screws go through. In this arrangement, in most cases, the distance between the screw holes is closer together on the frame, than on the heat bed. This means your screws come in at an angle towards the center.
If this is the case, then your first step should be to fully tighten the knobs, then loosen them off until they are almost off, and then move them to 1/2 way in between those 2 points. This is where you should start your leveling. If you start with the knobs fully tightened or close to that, you will find the center of the heat bed tends to bow updwards.
Because heat can alter the shape of the bed, we want the bed to be heated for at least 20 min before we continue, especially with glass beds. Te hotend should also be heated so everything is at operating temperature.
The next step in bed leveling is setting your z offset. Z offset is defined as the distance between the tip of your nozzle and the tip of the ABL (auto bed leveling) probe. If you don’t have an ABL, it is the distance between the nozzle tip and z=0, which is usually determined by an endstop.
To set your initial z offset, tell your printer to move the nozzle position to the middle of the bed, and home the z axis. Once there you put a piece of paper between the bed and nozzle. Now you adjust the z offset just until the paper is held tight enough that the you can feel it vibrate between the bed and nozzle, but loose enough you can remove and reinsert it. The vibration happens because of the heat and a tiny bit of filament at the tip of the nozzle. If you were to take a pencil eraser and hold a sheet of paper to the table with it while you moved the paper, it would have a similar feeling..
On some machines you can adjust the z offset in a menu, on other machines you have to manually adjust the endstop or endstop trigger before you home z. Once you have set your initial z offset, you are ready to begin leveling at the corners.
Procede to each corner and using the knob, turn clockwise to raise the bed towards the nozzle, and counter-clockwise to lower it. Again you are trying to get the nozzle close enough to the bed so that you can feel it held between the nozzle and the bed, yet loose enough to remove and reinsert it.
Moving any knob also affects the level of all the other corners, and the center, so you will need to go through all the corners until you no longer need to adjust it, then check the center. You are done if the 4 coners and the center are all have the same amount of friction when moving the paper between the nozzle and the bed. If you have an ABL, you can tell it to make an auto bed leveling mesh.
I highly recommend finding a 5 square leveling print. You make the adjustments to your knobs while it prints, to fine tune your first layer. A good clean first layer will give excellent bed adhesion, and provide a good foundation for the rest of the print.
You can find my YouTube video on this topic here: https://www.youtube.com/watch?v=M0SftHuvNms