Step 1 - You will need a radiator key and an old rag. Radiator keys are readily available from DIY stores and ironmongers.
Step 2 - Armed with the rag beneath, use the radiator key to slacken the air bleed valve which is at one end towards the top of the radiator. There will be a hissing sound as the air comes out.
Step 3 - As soon as water begins to flow, close the vent again and wipe away any water. Take care not to get scalded – the water may be pretty hot. The heating can then be switched back on.
Bought to you by BN1 Property Maintenance the Brighton Builders