travel from heathrow to Stansted

Paula....I would suggest that, depending on the time of day, you look at the coach service than runs between the two airports, taking into account the warnings about potential traffic delays during rush hours, etc. A good site to get the latest info, and check on schedules, can be found at http://tinyurl.com/39oyey . Be aware that Stansted airport is actually about 60 kilometers NNE of London, so you can't take the underground alone....It would mean taking the underground (or the much more expensive Heathrow Express train) to downtown, then taking a train or bus out to Stansted. It would probably be better just to catch the bus mentioned in the website I've listed.

According to the tube traveler,
The journey time is approx 90 minutes (110 minutes to/from Terminal 4)

The most important thing is to allow up to six hours between your scheduled arrival at Heathrow and your scheduled departure from Stansted. If you miss the check-in deadline at Stansted, your ticket will be worthless and you'll have to buy a new one for a later flight at a very high price.

The London Toolkit site given by Norm also has a suggested route through central London. See here. Note that while it's cheaper than the direct coach, it takes longer.

The absolutely cheapest route is to take either the Tube to Victoria rail station or a National Express coach to Victoria coach station. (The fare in either case is £4.) Then take the easybus from Victoria to Stansted. The fare varies depending upon how far in advance you book a ticket.

National Express bus service estimates it at between 75 and 90 minutes from Heathrow to Stansted. But, as noted above, time of day matters a lot. It's the easiest method, though -- no transfers. The bus runs between the Central Bus Station at Heathrow and the front door of Stansted airport.