In the height of summer there are many ways to get to Sifnos, there are lots of boat connections from different islands where there may be airports to speed your trip. But for a reliable all year round and straight forward connection to the island from Athens, simply go to the port in Piraeus and catch one of the daily sometimes twice or thrice daily ferry boats. There are also high speed Trimaran ferries which do the normally 4-5 hour trip in under three hours.In the winter months the connections are much less frequent though. Some information about ferries can be found here. http://www.greek-islands-ferries.gr/