You can fly direct to Skiathos (there are several Travel agents who sell only flight tickets).
You can fly to Athens airport ,and then get a bus to Ag.Konstantinos.
From Ag.Konstantinos you can get a hydrofoil that takes 1.30 hours to Skiathos, Or you can get the ferry that takes 2.30 hours
About Skiathos airport
Skiathos Airport ‘Alexandros Papadiamantis’ (IATA: JSI, ICAO: LGSK) is an airport on the island of Skiathos, Greece. Its 5,341-foot runway is able to accommodate aircraft up to the size of a Boeing 757-200. The runway is characterised as ‘short and narrow’. Because of the uneven terrain on the island of Skiathos, Skiathos Airport was created by reclaiming land from the sea between Skiathos island and the smaller island of Lazareta (a former leper colony) effectively joining the two islands into one larger island.
The airport’s short runway and its proximity to an adjacent public road have made it a popular destination for planespotters.
It is often compared to Princess Juliana International Airport on Saint Maarten due to the ability to experience low-altitude flyover landing approaches and takeoffs.