Turn off the heating system and let the radiators cool
Find the bleed valve on the radiator
Place a cloth or small container under the valve
Insert a radiator key or suitable screwdriver into the valve
Turn the valve slowly counterclockwise
Release trapped air until water starts to come out steadily
Turn the valve clockwise to close it
Wipe away any water
Check the boiler pressure and top it up if needed
Turn the heating back on and check the radiator heats evenly