Borrow from neighbour or buy a radiator key from a hardware store. Has square socket to fit square headed bleed screw at one end of radiator, at the top.
Undo screw about turn and a half and listen to the air hissing out. Eventually water will emerge. Close bleed screw.
Have rag handy to catch water and stop it staining walls and floor. Do not expect water to be gin-clear.
Peccavi, October 2008