Quickest route from Airport to Parc des Princes

If you don't have any luggage, here is the fastest and easiest:

After you get out of customs, head for the Metro/RER station (it's in the airport, just follow the signs).

Get on RER B heading towards Robinson (there are other names as well (St. Remy & Antony), but being an English speaker, Robinson is what you're eyes seem to see first. By the way, RER B is marked in blue)

At the St. Michel station, get off RER B and follow the orange "Correspondance" signs to Metro 10 heading for Boulogne/Pont de St Cloud (DO NOT HEAD towards Gare D'Austerlitz). (By the way, St. Michel is also the Notre Dame station and the Cluny station, the RER station will say St. Michel on the walls when you get off, but there is a long walk to Metro 10, so don't get nervous, just keep following the orange Correspondance signs to 10 towards Boulogne.)

Get off the Metro at Porte d'Auteuil and go down Marat Blvd (look at the map at the Metro station (if you don't have one yourself) and head away from the Eiffel Tower and the race track). (Actually, at this point, I suspect you could really just follow the crowds). Parc de Prince is on the right.

The whole trip should take about an hour and a half, but I wouldn't waste time because int'l flights are known to be late (and who knows what Customs will be like during the Cup)

So, just as a quick review:

RER B towards Robinson, change at St. Michel to Metro 10, towards Boulogne, get off at Porte d'Auteuil.