It's very easy. Take LIRR from Penn Station to Jamaica. Walk upstairs to AirTrain and take that directly to your terminal. There are people at the AirTrain entrance to help you. You'll pay $5 for the AirTrain plus whatever your LIRR fare is. There is also a bus from Penn Station to JFK but I think the train is faster given traffic.
It's very easy. Take LIRR