since you already know the basics of CSS , the next thing is deciding your subjects. as you might have seen you have to select optional subjects that make a total of 600 marks. since you are in georgetown i believe you have good grip on english and with daily newspaper reading you can cover current affairs too. decide your optionals , buy some books and whenever you have spare time read them

opt subjects like international relations, political science that you are already studying . this will reduce the workload. then once you are free , you can give your 100% to preparation and at that time i think 3 months will be enough for you.