Locate all of the bleed valves in your system. If your system is a base-board system you will have to open the door on the front to find the valve. The valves on the base board systems are generally located where the pipe comes through the floor. The valve on a standing radiator is usually on the top at the end. Take note of the type of tool that will be necessary to open the bleed valve.
Some valves use valves that are opened with a straight-edge screwdriver.
Some use a square fitting that requires a radiator key. Start on the radiator that is farthest away from the boiler. If your house has a second floor, start there. Place something under the spout. If there is no room, a piece of air hose from a fish tank sometimes works.