The preparation for IES starts with your engineering itself. First make sure that you have done all your courses thoroughly.start solving the previous year question papers of IES. That will be of great help. IES exam consists of a single written stage only i.e. there will not be any preliminary. the written exam will consist both objective as well as subjective stages and you have to score well in both. As you are in a job you will be to strategically distribute your time. Make sure you make a time table and stick to it, because after job you will get very little time to prepare so you have to make most of it.