Partially fill a blocked sink with water and place the cup of a sink plunger over the plug hole. Tightly pack a damp cloth into the overflow to prevent a loss of pressure.

Pump the plunger up and down vigorously and repeat if necessary.

If that fails, try using a chemical drain-cleaning product. Wear gloves and goggles and follow the instructions carefully. Smear some petroleum jelly around the rim of the plug hole to protect it from damage.

If you have not been able to clear the blockage then you will have to remove the waste trap.

Put a bucket underneath the trap to catch any spillage.

Unscrew the trap and empty the contents into the bucket.

Replace any washers or O-rings.

If there was nothing blocking the waste trap, use a drain auger to probe into the waste pipe.