Watch this video from This Old House to learn how to fix a bath sink stopper.
1 Pull out the old pop-up sink stopper.
2 Use an adjustable wrench to remove the pop-up lever from beneath sink.
3 Loosen the compression nut connecting the trap to the vertical drain pipe.
4 Use pliers to loosen the tailpiece.
5 Unscrew the pop-up drain assembly from the sink strainer.
6 Use two screwdrivers from above to prevent strainer from spinning; loosen strainer nut from below.
7 Pull out old sink strainer.
8 Place plumber's putty under new strainer; set strainer in sink.
9 Tighten nut from below to secure new strainer; clean away excess putty.
10 Thread new pop-up assembly onto strainer.
11 Cut tailpiece to length using tubing cutter; coat threads on tailpiece with pipe dope.
12 Fit tailpiece between trap and pop-up assembly; thread tailpiece into assembly.
13 Tighten compression nut on trap to lock tailpiece in place.
14 Insert new drain lever between pop-up assembly and perforated linkage.
15 Place stopper into sink drain hole.
16 Test stopper to ensure it glides up and down smoothly.