Leave the airport by driving South towards the main north coast highway, NOT West towards the town.
After about 1 Km. turn right up the ramp following the signs for Rethymno and you'll be on the main road.
Keep going for 83 Km. then take the slip road off to the right, signposted Spili then turn left at the stop sign at the bottom and pass under the main road.
When you get over the mountains, take the right turn signposted Prevelli Monastery (brown sign) and Plakias.
You now have about 15 Km. to go.
Then just follow the signs for Plakias, turning left just through the village of Assomatos.
Total distance, 117 Km.