To mow down long grass, follow the rule of thirds: do not cut more than one-third of the length of grass at a time as it could damage the roots and encourage the spread of weeds. Begin by trimming the grass enough to where it is standing upright with no bent blades before you start mowing. After trimming, adjust the mower to its highest setting to make the first pass, then gradually reduce the height of the blades until the mower is returned to its standard height, which is generally between 2 and 3 inches. Be sure to stop frequently so the mower does not get clogged, overheat or stall out, and allow a few days between passes for the grass to recover.