First we will need to set the Z height in the middle of the bed.
Place a business card in the middle of the bed, making sure that the X axis is centered (this can be done by hand if the power is off), and tell the printer to ‘Home Z’.
This is done via the LCD controls (Menu>Position>Home Z) or through Repetier (Manual Settings>Z Home).
The height should be set up so that the business card can be pulled out without tearing, but still have minor resistance. Adjust the Z stop bolt so that it is longer (raises the axis) or shorter (lowers the axis) until you have the desired Z Home height.
Once the middle is set we can move the extruder across the Y and X axis. Repeat the process on the furthest points on each axis using the business card method, but raise or lower the corners of the bed by tightening (raising) or loosening (lowering) the screws under the bed under the corresponding corners.
Wave a lighter flame under the filament for a few seconds about 20mm from the end. Stretch the filament a bit while it is warm as shown – do not snap it as the end will then curl. Allow to cool, blow, or use burnt finger as heatsink.
Trim end to a stout point. This will now easily feed through most extruder mechanisms.
There is a brilliant tutorial on how to calibrate your extruder here.