Look under the sink to ensure that the retaining nut is off the sink drain basket. Remove the securing nut from the sink drain basket with a pair of channel lock pliers. If the sink drain spins while you are removing the nut, you will need to secure the sink basket.
Ask a friend to hold the handles of a pair of pliers inside the sink drain basket. This will keep the sink drain basket still while you are removing the retaining nut. If necessary, spray some penetrating oil on the threads and allow the oil to sit for one hour. Go back after the hour and remove the nut from the sink drain basket.
Cut around the perimeter of the sink flange between the flange and the sink with a utility knife, if the sink flange is still stuck to the sink. This will score some of the plumbers putty under the flange.
Go under the sink and hold a small board against the bottom of the sink drain basket. Tap the board with a hammer just enough to break the seal of the plumbers putty.
Ask your helper to slide a putty knife between the flange and the sink while you tap on the wood. Have your helper work the putty knife around the sink flange until the flange releases from the sink.