Getting here
This page will tell you how to reach Koskina village from Athens International Airport. If you are starting from somewhere else, you will need to make the necessary adjustments. Once you reach the village, follow the further instructions you will have been given.
There are several alternative routes you may follow. All of them have a journey time of approximately two hours (plus any waiting times for ferries).
1. All the way by road
Take the highway that runs from the airport across the north of Athens (Attiki Odos; about 15-20 minutes) and turn on to the main north-south National Road leading to Chalkis, Lamia and Thessaloniki. Turn off at Chalkis, take the suspension bridge to Evia and then drive south along the coast.
If you choose this option, you may want to stop and do some shopping in Aliveri, which is about half an hour before Koskina (near where the journey time callout is rooted - zoom in on the map to see more). There are well-stocked supermarkets outside the town, near to the ring road: MyMarket on the left as you come in to Aliveri before turning right on to the ring road, 
on the corner where you leave the ring road, and Vassilopoulos (AB) on the right, about half a kilometre after that turn.
2. Ferry options
Skala Oropou to Eretria
Start as in the first route above, but leave the National Road at Malakasa (there are two intersections, so if you miss the first one you can take the second) and drive down to Skala Oropou. Car ferries leave every hour (but check the timetable - see below) and cross to Eretria on the island in twenty minutes. From there you resume the same route as in the all-road option above. This also lets you shop in Aliveri if you wish.
Check the Greek Travel Pages for ferry availability and timetables.
Agia Marina to Nea Styra
This one is quite different. You turn off the National Road after only a few minutes and drive through Marathon to Agia Marina harbour, from where ferries leave several times per day for Nea Styra.
If you're aiming to catch a ferry, allow at least an hour to get to the port, whatever Google maps or GPS tells you. The final turn from the back country road down to the harbour is signposted, but easy to miss, so watch out.
The crossing takes 45 minutes. There are shops and restaurants in Nea Styra, from where it is half an hour's scenic drive to Koskina.
Check the Greek Travel Pages for ferry availability and timetables.
Both departure harbours have cafes and restaurants where you may sit if you have time to kill. The ferries also provide beverages and snacks on board.