Turn off the central boiler on the radiator system. Most boilers will have an "On/Off" switch attached somewhere to the boiler.
Loosen the bleed screw on the leaking radiator with a radiator bleed key or standard wrench. Pull the screw out of the radiator. Wrap the threads on the screw with plumber's tape. Insert the screw into the radiator and tighten.
Turn off the water supply valve for the radiator. These usually take the shape of knobs or nuts. The supply valve may be hidden beneath a plastic cap that must first be removed before you can turn off the valve.
Place a bowl underneath the radiator drain valve. These are usually located on the inlet side of the radiator. Loosen the drain valve and let the water inside the radiator drain into the bowl.
Loosen the coupling for the outlet valve at the opposite side of the radiator.
Wrap the coupling threads on both sides with plumber's tape. Reconnect the couplings and tighten with a wrench.