Turn off the heating system and let the radiator cool down
Find the bleed valve at the top side of the radiator
Place a cloth or small container under the valve
Insert a radiator key or suitable screwdriver into the bleed valve
Turn the valve slowly counterclockwise until air starts to escape
Wait until the hissing stops and water begins to come out steadily
Tighten the valve clockwise as soon as water flows out
Wipe away any spilled water
Check the boiler pressure and top it up if needed
Turn the heating back on and confirm the radiator heats evenly