1) Your first check should be the water level in the cistern is at the correct level normally marked on the side.
2) Inspect the float valve is working correctly and cutting the water at the marked level. Also check the round float (if fitted ) is not punctured.
See post no 15 if you need to replace the float valve which is the same principle even though they are different.
3) Check AAV (if applicable) is functioning correctly and allowing air in.
4) See if your siphon is in 2 parts and allows the diaphragm section to be withdrawn. You can then check the condition of the diaphragm (thin plastic sheet on the bottom of siphon) and replace if torn.
5) If you have a single part siphon :
a) Isolate the cold supply to the cistern and flush the WC. Sponge (or wet vac) out the remaining water.
b) If close coupled then remove cistern from pan and wall, if a low level detach flush pipe from siphon.
c) Unscrew back nut and remove siphon.
d) Install new siphon or just replace the diaphram.
e) If close coupled then ensure that you use brass screws when reattaching the cistern to the wall, and consider using a new doughnut washer. Reattach flush pipe if low level WC
f) Reinstate cold supply and test.
