This is an excerpt from the Mendel90 build instructions, which are work in progress, but it can be applied to other machines.
The best way I have found to level the bed relative to the nozzle is to use a dial gauge mounted in place of the extruder using this clamp. If you don't have a dial gauge you can roll a rod or slide some film under the nozzle and feel when it is just touching.
The two pillars at the back of the bed have a washer under them to ensure the front can be made both higher or lower than the back. Those pillars are tightened and not adjusted. Front - back adjustment is achieved by adjusting the two pillars at the front. Left - right adjustment is done by turning the Z lead screws. Ideally there would be only one mounting point at the front as only three points are needed to mount a stiff sheet like glass. Having four makes the adjustment more tedious as they tend to bend the sheet and interact with each other.
Start by sliding a washer under the front pillars to set them to the same height as the back. Move the gauge or nozzle to the middle of the back of the bed. Note that level on the gauge, or nudge the Z axis to just touch your feeler. This is the level that we want the whole bed to be at.
Move to the back left corner and adjust the left lead screw to get the same level as the middle. Move to back right and adjust the back right lead screw. Moving the gauge all the way across the back should now read the same height.
Now move to each of the front corners in turn and turn the pillar until the height is correct. Lock them in place by tightening the top screws through the bed.
The whole bed should now be level but usually you need to repeat the procedure a few times due to the interaction of the four points.