From JFK to Grand central there is s direct bus service.
this is easy and costs 15 USD one way. (Go outside of terminal and look for big blue busses at this moment they carry advertising for Jetblue)
Cheapest way : take airtrain to Jamaica station, (5 USD)
than take E line of the New York subway. (2USD)go to times square than transfer to the shuttle to Grand Central terminal.
transfers are free.
Very easy : Taxi now 45 USD + tolls and tip
From JFK to Grand central