Do not tighten down all the screws yet, just attach the rail by the two screws on the ends - later adjustment maybe required

Attach the Y MGN 12 rails on both sides of the printer

Use the 3030 corner bracket in the back as reference - the rail should touch the bracket

If you don't have the MGN Alignment Tool, measure the space between the side of the rail and the side of the extrusion - for 3030 it should be 9.00mm on each side - measure each rail in 4 points