The donut washer is squashed down to form a seal by a flat plate - the plate is pulled down by a bolt/wing nut on each side - at the back underneath the pan.
Sometimes by simply tightening the wing nuts by hand you can stop the leak.
Even though it may appear difficult it's a relatively simple matter (10 mins) to remove the cistern to put in a new donut washer.
Turn off water - remove cistern lid - flush loo - disconnect water inlet pipe - remove two wing nuts (as mentioned) - remove two screws holding cistern against wall - carefully lift cistern away from pan and tip remaining water into the pan.
When reassembling do not screw the cistern too tightly to the wall else it may crack.
Good luck...
Peccavi
September 2009