Dear!
First of all, read the whole CSS syllabus carefully and select the subjects of your interest, candidates always want to pick those subjects which are thought to be more scoring but remember that it always depend upon the candidates that how much marks he can score. All optional are scoring if the candidate have prepared it in an efficient manner.
Start your compulsory subjects first.
Go through all the past papers, compulsory as well as optional and extract an aggregate of the important questions which have been asked repeatedly and prepare those very questions to the point of mastery.
Good Luck.

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"Success is not final, failure is not fatal; it is the courage to continue that counts" (Winston Churchill)