How hard is it to travel from Naples Airport to Praiano ?

One way is to take the bus to Napoli Centrale station, go downstairs to the Circumvesuviana (buy your ticket before you go downstairs), take the Circumvesuviana to Sorrento (last stop, make sure you get on the right train and be vigilant about personal belongings), at the Sorrento train ststion go outside and take the bus in the direction of Amalfi, get off at Praiano. This will save you money but not time. Bus leaves the airport at least every hour, if not more frequently, Circumvesuviana takes a little over an hour to Sorrento, bus to Praiano can take an hour or more, depending on traffic. Total cost on public transportation should be less than 20€.

You can check arome2Rio.com to compare transportation means.

You could be met in Sorrento, which should cut the cost nearly in half.

In addition to Zoe's suggestion, there is another option that might be more comfortable. The bus company called Curreri Viaggi operates a bus from NAP airport to Sorrento. I think that option is better than going through the Central Station at Naples and then the Circumvesuviana train. The cost is only €10 pp.

This is the timetable (I can't open the Curreri Viaggi website for some reason):
http://www.sorrentoinfo.it/orari-autolinea-aeroporto-di-napoli-capodichino-sorrento/

Once in Sorrento you would take a SITA bus to Amalfi and get off at Praiano.
http://www.sitasudtrasporti.it/public/tpl/campania/5070.pdf
Didn't check the prices but I'm sure it's less than €5.

It's about 2.5 hours, instead of 1.5 with a car, but the price can't be beaten.

Roberto is right, I forgot about Curreri.