(modified from the article about Paris)
You might want to go to the service station Chartres-Gasville on the A11 east of Chartres (bus plus one hour walk!). The service station is found on both sides of the motorway, they are even connected by a bridge, so you can hitchhike in both directions.
From the train station, take one of many buses that stop at the Morard station. Here starts bus 12, take it and get off at N.Conté - some minutes before you arrive there, memorize the route: the bus crosses the motorway, and comes to a big roundabout where it turns right and continues to a second small roundabout to turn right again to arrive at N.Conté. Now comes 1 hour of walk: go back to the big roundabout and turn right (east) to follow the N10 until you come to a small city. There you encounter the next roundabout, where you turn left. Follow the street, it bends to the right and ends on the Rue du Bois Paris where you turn left (north). After 500 meters comes the motorway.
If you want to go east to Paris, go to the motorway but don't cross it but pass through the trees on your right and continue to the service station (400m meters).
If you want to go west, you have to continue until you have crossed the motorway. The service station is 400 meters east, so try to pass through the trees on your right and follow the street to the service station.