In short
Plakias: easy, several buses a day (06:00-19:30), ~40-50 min, ~€5. Preveli: trickier — 2 departures (10:30 and 16:30) with a change at Plakias, returns 12:00 and 18:00, in season. The bus drops at the monastery; the beach means a 15-20 min descent down steep steps.