Badman tells Niko to go to Wallkill St in South Bohan. Stop in the marker and watch the cutscene. Follow the dealer on foot, being carefull not to spook him. If the dealer does get frightened, he will start running. This will not fail the mission, but it is more difficult to keep up. Roman will call during the mission, and the phone may scare the dealer, so hang up the phone as soon as it rings. Follow the dealer into the apartment building, but don't kill him.
The dealer will meet his suppliers on the top floor of the building. The door may be locked, depending on whether or no you spooked the dealer. If it isnt, push the door open and start shooting; if it is, shoot the lock to open the door. Kill the three people inside. After this, Niko will call Little Jacob and tell him the dealer and his suppliers are dead.
Tip: Before following the dealer, be sure to set your cell phone to "quiet" because you'll get a call from Roman as you follow the dealer, possibly spooking him and losing your element of surprise.