Why this happens

Family Handyman says a puddle in the cabinet under a kitchen sink may be caused by a leaky basket strainer. The seal is made by plumber's putty between the strainer flange and the sink, plus a rubber gasket, a friction ring and a locknut under the sink. Kohler and Elkay show a similar layout: putty at the flange, and a rubber gasket or washer under the sink that is tightened in place.

How to fix it

This page is for a bowl with no garbage disposal. If the leaking bowl has a disposal, stop here and call a plumber. Bob Vila says that resealing a disposal's sink flange means detaching the disposal first.
If a plug, cord or outlet under the sink is wet or sitting in water, stop here. Do not touch it, do not add water, and call an electrician. Turn off its breaker only if you can reach the panel safely.
Test first. Close the stopper, fill the bowl a few inches and touch a facial tissue between the bottom of the strainer and the sink, as Family Handyman shows. If the tissue picks up water, the strainer is leaking. Family Handyman says this test tells a leaking strainer apart from a leaking pipe joint.
If a chemical drain cleaner has gone down this sink, stop here. Do not take the drain apart. Call a plumber and say what you used.
Let the bowl empty. Shut off the water supply, as Kohler's installation guide says, and keep the faucet off. If a shutoff will not turn, do not force it. Stop here and call a plumber. Do not take the drain apart with the water still on. Set a bucket under the drain pipes.
Loosen the coupling nut at the base of the strainer with slip-joint pliers and slide it off. Family Handyman says that if the basket spins, you hold it still.
Unscrew the locknut with slip-joint pliers or a spud wrench. Take off the locknut, the friction ring and the gasket, and lift the old basket out. If the locknut will not turn, stop and call a plumber. Do not cut it or hammer it.
Scrape the old putty off the sink with a plastic putty knife so you do not scratch the sink.
Roll plumber's putty into a rope about the size of a pencil and wrap it around the lip of the drain opening. Press the strainer firmly into the putty, then add the rubber gasket, the friction ring and the locknut under the sink and tighten the locknut. Family Handyman says the cardboard friction ring goes between the gasket and the locknut so the locknut turns freely.
Family Handyman says putty cannot be used on certain new sinks, and those sinks are labeled that way. In that case it says to use a non-water-based silicone. Read the warning label on the putty itself before you use it.
Wipe off the putty that squeezes out. Reconnect the coupling nut and any drain pipe you moved, open the water supply, then fill the bowl and check that the underside stays dry.

When to stop. If the locknut will not turn, the basket spins and cannot be held, the sink is cracked, or the drip continues after you reseal it, stop and call a plumber.

If this does not fix it, tell the plumber

A drip forms right under the strainer where it passes through the bowl.
Whether the tissue test picked up water at the strainer.
Whether the locknut turned or was stuck, and whether the basket spun.
What kind of sink it is, such as stainless, composite, stone or porcelain.
Whether you resealed it and whether the leak came back.

Build the full list: what to tell the plumber.

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References (every fact above comes from one of these pages)

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