Download previous 3-5 years' question papers from the IGNOU website, depending on your time constraints. Look for answers to these questions in the IGNOU material. It is pointless to recommend guides because the majority of their answers do not adhere to the topic or syllabus.
Make a list of the most important points. Remember to take notes on the keywords. Highlighting these keywords while writing the exam will give the examiner the impression that you have a good understanding of the question.
Depending on the question, try to include some diagrams or maps. It will undoubtedly give you an advantage. Because you have not read the entire material, you may find it difficult to understand at times. In that case, Google the keywords or the context to get a good idea of what the problem is.
Prepare each previous question paper in this manner and revise it at least twice. Examine your assignment question as well.