From Athens: Once you arrive at Athens airport just outside the Terminal on the right you can purchase bus tickets from the white ticket office. The bus number to Pireaus (the port of Athens) is X96.
Don´t forget to validate the ticket on the bus. The travel time is 1 hour and 15 minutes.
A ship travel with Flying Dolphin or Flying Cat takes 1 hour to Poros.
A ship travel with regular ferry takes 2,5 hours.
You should buy your return tickets as soon as you arrive on Poros.
When you return to Pireaus on your way home you look for bus X96 to the airport.
A good option if you arrive late to Athens and do not have time for the last boat is to rent a car and drive to Poros via Corinth.
It is possible to rent a car one way with POP's Car rental.
Option two is to stay one night in Pireaus and continue to Poros on the second day.
Directions from Athens to Poros, see page Rent a vehicle
The guide of Poros