I could have sworn that I did a re-cap on how I got my French driving license but looking back through my posts it does not appear that I have...
(this is all based on the fact that I have my American license from a non-exchange state).
Step one: Stop by any Auto-Ecole and ask for the formulaire 02 - the PDF can be found here - to take the code de la route (written exam). You can either sign up to take it with them (and PAY!) or you can send the filled out form and send it to your local Prefecture to take it as canidat libre (along with the documents required, ask your auto-ecole for the list but if I remember: 3 ID pics, self-addressed stamped envelope, justificatif de domicile and photocopy of your American license). I also included a letter asking to take the test for non-Francophones. This option exists at least in my department and is held every 3 months in the head office. You can take the exam for free if you do it directly through the Prefecture. It might take longer to get a date - especially in cities like Paris but if you send off the document first of all it will give you time to study while you wait.
Step two: Study STUDY STUDY! Pick up one of the Code de la Route DVDs/iPhone apps etc and practice as much as you can!!! Everyday if possible. The questions are utterly stupid (although I've heard there is a movement to make the more related to actual driving...). You think you know how to drive but you don't know how to answer their stupid questions so practice as much as possible!
Step three: Take the test. Go to your convocation, breathe deeply and don't stress like I did the first time.. boo.
Step four: If you pass the code then go back to the Auto-Ecole with the stamped document that they gave you and ask to sign up for a few hours and the driving test. Each hour will run you between 30 - 40 euros. If you already know how to driving a stick good for you! Will be much easier and I bet you'll only need 2 or 3 hours. I did a total of 9 hours because I was only just learning how to drive a stick and I had not been driving in France on my American license so it was all news to me. They will sign you up for the driving exam and you'll get your convocation.
Step five: Take the driving test. Some smaller towns still tell you if you passed right there after the test, others will send you the notice in the mail. Once you pass the test you'll have a document that allows you to drive on it until you get your license. Once you get your license in the mail you're good to go! (for the next 3 years, you'll be considered a "young/novice" driver until then, only 6 points and you're technically supposed to put the "A" on your car and driver under the speed limit.) Technically you can also take the driving test as canidat libre but you have to provide your own car which has pedals on the passenger side... I looked into renting one but it would have cost more than doing it with the Auto-Ecole. Honestly who owns a car like that??
It took me about a year start to finish for a couple of reasons. The first time I sent in my request to pass en candiat libre they did not have a complete application and instead of asking me they just threw it away... 2 months wasted. Then I failed the first "code" exam since I totally stressed out. Waited another 3 months and took it non-Francophone the second time around, MUCH better! Then I failed my first driving test and had to wait for a second date a couple months later. It also cost me a total of 430E (9 hours plus the cost of taking the driving part twice - the 2 written exams were free as I did it directly through the prefecture.)
I hope I'm not forgetting anything and I hope this helps!