You should travel from King's Cross St. Pancras underground station
on the Victoria Line to Tottenham Hale, and then change to a WAGN
train to Stansted Airport.

But bear in mind that they charge 22,70 £ (24 £ from Liverpool Street)
return. So if you are not in a hurry the airbuses to Victoria and Stratford
at 12-15 £ return might be better value. http://www.nationalexpress.com/

But I have a question concerning these buses: Is it advisable to book ahead?
At my local airport (Köln, Germany) the buses run frequently, you just take
the next bus and pay at the driver. Booking is neither necessary nor
possible. The National Express web site on the other hand offers booking for
the airport services. So I don't know if it has advantages (even if an air
plane might always be late or early) to use the reservation service. Your
experiences are welcome.

Thank you.

Martin