If you haven't started preparing yet, you can do so starting today. To get an idea, look at the previous year's question pattern and thoroughly familiarise yourself with the RRB Group D Syllabus.
The next step is to devise an effective study strategy for which topics will be studied on which days. This will assist you in more effectively preparing.
After you've finished each topic, put your knowledge to the test by taking RRB Online Mock Tests. This will give you an idea of how well you are preparing and whether you need to focus more on that specific subject/topic.
When taking the Online Mock Tests, keep accuracy in mind at all times. Each time you take a mock test, try to improve your speed and accuracy; this will help you achieve your goal of being hired for the RRB Group D Post.