1. Start by booking your studio tickets. These seems to sell fast and should be booked at the start of the planning process.
2. Now that you have your tickets, it is time to sort out how to get there from Heathrow. Although it is only about 20 miles, I've found that taxis in London can be both slow and very expensive (traffic). There are two public transport options that I've found.
a. Rail Service
It seems as though each ticket is about $40 and the trip takes roughly an hour. You can view schedule etc. on the national rail website
Getting to the train station from LHR is pretty easy - there are bus/shuttle services that will take you there and there are manned ticket stations where you can ask about tickets.
b. A better option (although I haven't taken it yet) seems to be the easy bus - for a whopping $2.00 per person you can take a direct service no transfer bus from Heathrow straight to Watford.
Although a bus isn't as romantic as a train - the price and lack of transfer makes this a pretty appealing option...
The studio tour website says there is a free shuttle from the train station to the studio - I'll post the details after we've made the trek.
3. Book your evening stay in Watford
As we will be flying into London from the US, we'll be plenty tired by the end of the day - the tour is supposed to take 3 hours (we'll see...) so I think our best bet will be to stay in Watford proper. The best options for hotel can be found on trip advisor here:
We've chosen to stay at Holiday Inn express.
4. Figure out how to get around before you get there.
For this I love google maps - particularly the street view.
View Larger Map
I strongly advise using the street view option to get the "lay of the land" before you go over - this makes it much less disorientating after you arrive. You can literally walk the route to your hotel before you even get there.
Good luck and enjoy your trip!