Call the Korinthos Canal Authority on VHF 11 before leaving your anchorage and they will advise you what to do; just follow their instructions and be patient. If you happen to be behind a large ship hang backup a little (about 150ft) not to feel the prop wash.
Once you reach the east entrance tie up to the dock on the on the south side and go ashore to pay the canal fees. While there you can top up your fuel, the gentleman in the office will call for you. Or if you have your own phone you can call Kapsalakis cell +6944 3933 01, office 2741 0240 24, I paid Euro 1.48/Liter in 2011.