The quickest route is to turn sharp left past the Juice Bar on the beach road (just opposite San Stef Travel - otherwise known as the abode of the Three Graces of Agios Stefanos). Just keep going up past Tsaros - it's not that bad - and then turn left at the T-junction. (If you don't turn left or right you will fall 100 feet into the sea). Small rise takes you to Akrotiri. Try the smoked salmon salad - it's fabulous.
If you are (sensibly) staying on the old village, there's a nice, reasonably gentle, route. Turn left up the hill just before the church on the harbour road and follow the track. This takes a right turn and brings you out at the top of promontory, and you can stroll down with great views until you reach Akrotiri. Did I mention the smoked salmon salad?
There's a variation on this route for the impatient. Where the track turns right, go up the hillside, squeeze through the fence gaps, and you will end up in Arianna's. This is a hut and gardens, where you can buy a much needed drink. You may well encounter the lovely little girl after whom the enterprise is named. She has an English mother, and you will not need to speak pigeon Greek. She may well dance, sing, recite and generally perform her best acts for you. If you're a primary school teacher on holiday I'd give this one a miss, otherwise, it's lovely.
Akrotiri is a beautiful bar, and in our opinion one of the treasures of the village.