This is an old thread, but it's linked to the STICKY thread, so I'll add this as a useful relevant reference:
"Leveling" your printer
A more detailed but similar process:
level the X carriage bars (This section may not be required)
Consider leveling your bars as 'step 0', and worth at least eyeballing to check for level prior to adjusting the bed.
Note: You may NOT need to do the above steps every time, but because z-slouch can happen after even the slightest vibration or touch... it's worth checking each time, even if all you do is eyeball that the lower bar is parallel to the case. I home the printer, pull the hotbed all the way forward, and with my eye up 30ish cm over the front of the hotbed and in the centre of the bar, compare the gap on either side between the bar and the rear edge of the case. If the gap looks the same it's probably still fine and you can omit the previous steps.
Each time you do this you will get better at it, eventually it's easy but always a little annoying.