After making sure the bed is level, adjusting the height of the first layer is the next step. To do this, place the nozzle over the center of the heated bed. Slowly lower the nozzle at 1mm increments until the nozzle lightly touches the glass. If the Z axis microswitch did not activate, raise the nozzle up and turn the height adjustment screw clockwise 1/4 turn. Now lower the nozzle again and repeat the process as necessary until the space between the nozzle and the glass is approximately the thickness of two pieces of paper. Do not become overly concerned with the height, just make sure that the nozzle DOES NOT hit the glass when it homes on the Z axis. After making your first print, you can adjust the height adjustment screw to dial in the perfect first layer thickness. Also keep in mind that the thickness of glass may vary from piece to piece, so it may be necessary to adjust height for different pieces of glass.