Drain cleaner is very simple to use. All you need to do is pour the required amount (this will be clearly outlined in the usage instructions on the label) down the drain without diluting it or running any water. Leave it for the recommended time for the cleaning product to dissolve the blockage. You can then run the tap to flush the drain out with water. You should find that the blockage has cleared and water drains away normally again.
You should always follow safety instructions when using drain cleaner, as it can be harmful when it comes into contact with the skin. For example, protective vinyl, rubber or nitrile gloves should be worn. Drain cleaners and unblockers should always be closed after use and kept out of reach of children and animals.