Go to Gare des Invalides Metro and RER station (or the nearest RER C station around your hotel). Make sure you arrive 15-20 minutes before departure so you will have enough time to buy tickets and find the platform. Take RER C (VICK or VITY trains) toward Versailles and get off at Versailles Château - Rive Gauche train station, it will take you about 40 minutes to get there.