In the box from the reel there should be a small plastic bag containing 4 small bits of plastic.
They are the centrifugal brake blocks .The red ones are the ones to start with .To fit them undo the screws on the left hand sideplate and lift it off.
Once you have done that you should see the end of the spool with a steel pin running crosways across the axle.
Place one block on each side and reassemble.
gives you the general idea
Have a read of http://neilmackellow.sea-angler.org/reel_tuning.html
the tuning for the 20 and 30 works pretty much the same
the blocks also have two sides the flat side and a domed side the flat side has more braking the domed has less.