It's neither easy, nor cheap! Air Rarotonga has only a guideline schedule and you need to visit their website to find out when and if they are flying. Even then, flights can be cancelled at very short notice if there aren't enough passenges to make the trip viable, or if there's insufficient fuel at Penrhyn to enable the return trip. It takes four to six hours to reach the island. And you'll pay around NZ$1,6000 for a standard return.
Alternatively, you could try getting there by sea, either from Rarotonga or one of the other northern group islands (assuming you can reach them as well). There are inter island vessels, if you don't mind very small boats! If you want to explore that possibility, speak to the harbour master at Rarotonga for information on sailings, which are few and far between.