Get on the number 4 train--magenta line on the Metro map--towards Clignancourt. No need to worry which stop to get off because you want to get to the END of the line. So sit down, make yourself comfortable and try not to look too much like a tourist. ( Check out how to use the Paris Metro)
When you get out of the Metro stop, you will have to cross a street to get to where the stalls are. This is already part of the flea market, but NOT the flea market you're expecting. Walk through all these stalls selling knock-off sneakers, sweatshirts, gloves, etc. Walk straight on, one loong block.
You'll have to walk through stalls like this. Resist the urge to even take a peek, it will be a waste of precious time.
Then you will come to this, an elevated highway. Cross under it and you're almost there. (A regular poster on the Fodor's online travel boards posted up a whole bunch of photos of his day at the flea market. Check it out here.)
Finally you will reach Rue des Rosier. Turn left.
You are now in the main artery of the flea market, from which all the individual markets branch out. There are over 2000 vendors in 12 markets here. To your right you will see an entrance to the Vernaison market. Vernaison is only one of twelve markets, but this is the place you will probably pick up bric-a-brac or smaller souvenirs. Paul Bert and Serpette are favorites of interior designers--bigger furniture, chandeliers, etc.
Voila, you are at Vernaison! Lot of vendors with amazing stuff.
Each of the markets has it's own map, displayed somewhere near the entrance of the market. You can also get a copy of the flea market map from any vendor. They will be more than happy to hand one to you.
When I first saw this map, it didn't mean anything to me. I couldn't imagine how huge the flea market was. You really have to be there in person to get your bearings and relate it to the map. Now I look at the map above and know exactly where to go next time!
Check out Theadora Brack's flea market tips. She also recommends visiting Port de Vanves in the morning, and then taking a cross city bus all the way to Clignancourt in the afternoon, a wonderful full day at the fleas. Make sure to start early!