Just a final assurance you have lots of time to make this connection.
Do you have any checked luggage on the transatlantic flight? That might hold you up a bit if you do.
Once you deplane you have to go through passport control. Just follow the signs. The process is reasonably quick but can be slowed down if all three Transatlantic flights all arrive at the same time.
Now you walk to the arrivals door. There is a sign that says nothing to declare and another one that says Declarations. Presumably you have nothing to declare so walk through that door and you are out of the secure area and into the lower arrival terminal. There will be a crowd just outside this door with private drivers, and tour representatives. Ignore these people and turn left and walk down the corridor a hundred meters or so until you find the escalators. There are also elevators but the Escalator is easy to spot. Go up to the top and you are now in Departures level where you will find information on the flights and the gates plus all the check in counters. The transition from Arrival level to Departures level will take you less than 2 minutes from the time you walk through that no declaration door.
Aegean airlines allows you to get your boarding pass 48 hours before your flight. If you have done that there will be no need to go to the check in counter. As I recall there are electronic machines in this area where you can get the luggage tags for your bags. Print them off attach them to your bag and find a baggage drop off location. They take bags for all the airlines at a central area. Its easy to find. Chances are you will be too early to see what gate your flight will be assigned to so you can wander the departures area. There are shops and restaurants and even a small museum of the artifacts they discovered when they built the airport.
When you are ready go through security which is a fairly fast process as they have lots of machines. Again there are lots of shops in the secure departure areas. You do want to be at your departure gate well before departure time. The reason is that domestic flight do not depart from the gate. You have to take a shuttle bus out to the aircraft which takes some time.
Hope this helps