Step 5 · Remove the Old Drain Assembly
With the faucet removed, focus on the old drain. From underneath the sink, use channel-lock pliers to unscrew the large slip nut connecting the drain tailpiece to the P-trap. Then, unscrew the large mounting nut that holds the drain flange against the sink. Once this nut is removed, push up on the drain assembly from below and pull the old drain flange out from the top of the sink basin.