Mix two spoons with a cup of warm water and pour it down the drain. Then leave it undisturbed overnight. This method only works if there is some drainage, and not in standing water. The procedure must usually be repeated three times, sometimes more, before the drain is clean again. I have used it twice with success, though more patience is needed.
Some hardware stores sell a special hose to connect to a faucet.
The other end of the hose has nozzles that produce high pressure water sprays. The nozzle end is pushed into the drain where it can sluice its way through the gunk. An ounce of prevention is better than a pound of cure, as Benjamin Franklin said. Before you notice any slow drainage, pour boiling water down the drain, or do a single treatment with the enzymes. This will keep the drains in good shape.