To tighten your bathroom sink drain, follow these steps:
• Find the large retaining nut under the sink. The nut secures the horizontal pivot rod to the back of the drainpipe
• Grip the nut with a pair of pliers. Carefully tighten it.
• Pull the plug handle upwards
• Turn on the water to see if the plug seals tightly against the drain.
• If it is still loose, tighten more.
• Keep tightening or losening the nut until you get the proper resistance.