Well, CDS has a vast syllabus. So, start with a well-thought and well-researched study plan.

Have a strong hold on the fundamentals of English and Mathematics subjects. These will help you cover most of the exam or scores. Prepare short notes for the tips and tricks. Refer to Word Power Made Easy by Norman Lewis, or Plinth to Paramount by Neetu Singh for the basics of the English language. Do not leave any topic of Mathematics untouched.

As far as the General Knowledge section is concerned, read NCERT books for the general science part. You can also go for Lucent, Spectrum, etc, to cover the Polity, History, Geography, etc, subjects. Reading the newspapers and editorials to keep up with current affairs, goes without mentioning.

It is going to be a long, overwhelming, and exhausting journey. So, you must prepare mentally to stay dedicated, motivated, and positive, no matter what comes.