Assuming the next year preliminary exam will be held in January, 2020, you have only 6 months left. So first you should divide your preparation strategy into 3 parts.
Basics Done : Getting basics done in history, geography, polity, economy, Current affairs from standard book. Also make sure not to go for more than one reference book for a palrticular subject as you don’t have much time to complete massive booklists. For example, in Polity follow Laxmikant book only. For geography go for Kartick Chandra Mandal’s Bharat o Paschimbanger Bhugol, in Economy of course Ramesh Singh is best and for History buy Krishna Reddy book. And try to complete all these theory part in 3 months. This is going to help you in prelims and compulsory papers of mains exam.
MCQ Step: After going through books try to solve at least 5 years preliminary papers and compulsory mains question papers within one week to get a clear idea of question pattern. And since now practice as much mcq as you can from various practice sets available online and offline. You can also do upsc general studies practice sets from various apps. One of the best and cheapest application from PrepNet.
Continuous Rank Monitoring : Any one can get best result by monitoring his / her performance. Without any proper system you are unable to judge your position among all the competitors. That is why you need a dynamic ranking system. you can easily get this facilities at www.prepnet.in
Exclusively for Mains : Now start preparing for optional paper. It is the optional paper that will give you better rank and group A or B service. But start focusing on preliminary exam from mid of December as you will get more time for Mains after preliminary is over.