how do i get rid of crabgrass properly?

Crabgrass is an annual, so the whole game is stopping next year's seed rather than killing this year's plants. A pre-emergent goes down in early spring, before the soil warms up enough for it to germinate - the old rule is when the forsythia starts flowering. Miss that window and it does nothing. For what you've got now, pull the young clumps before they set seed, and get the lawn thicker so there's no bare soil for it to land in. Mowing a bit higher helps more than people expect, the taller grass shades the ground.