Step 1) Turn of your heating. Doing this will prevent you from burning yourself and also stop the floor from being soaked with excess water. I do also advise putting a towel down when bleeding your radiators.
Step 2) Find the release value. It’s generally located on the top and on the side of the radiator and is identifiable by the groove that will match the radiator key.
Step 3) Twist the key anti-clockwise, releasing air in the radiator. It is important that you do this with a cloth or bowl ready to catch the dripping water. As the valve is turned, there will be a hissing sound as the pressure is reduced, indicating that the process is working.
Step 4) Once the hissing stops and water begins to trickle out of the valve, tighten the screw clockwise, making sure not to over-tighten and cause damage.
Step 5) Wipe any excess moisture off the radiator to avoid rust build-up.
Here is a link to help with more detail.