Print this to aid in leveling the bed. Make small adjustment when the extruder is at each of the four corners. Remember to only make adjustments to opposite corners first (do not go from bottom right to bottom left for example) or you'll be leveling all day. You have the right height when the lines are touching but not overlapping. If there is any gap or your deposition path appears cylindrical and not squished your nozzle is too far from the bed. Make sure to turn off your skirt before printing as this will outline your bed.

This part is 180mm x 180mm, the walls are 1.5mm wide and it is 1mm tall.