its too slowly increase the pressure on the head all around rather then tightening it fully on one end and having the other end not bolted on which could cause damage
Right you ahve to tighten the bolts in a certain sequence and in stages otherwise youll mess up the head. So tighten the bolts to 30 NM (1st stage) then tighten them again another 90 degrees (2nd) then another 90 degrees (3rd)