Leave your lawn untouched without mowing it for two days to prepare it for the weed killer formula.
Shake up a bottle of Spectracide Weed Stop for Lawns Plus Crabgrass Killer and adjust the nozzle to the "on" setting.
Saturate all of the weeds you want to kill with the spray, making sure to cover the entire weed, from root to top.
Spray the weed killer solution over any crabgrass, again making sure to saturate the blades in their entirety.
Wait for two days for the Spectracide formula to work, still refraining from mowing.
Remove and discard all of the dead weeds and patches of crabgrass in a yard waste container.