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First of all, you need to have good knowledge of English grammar. 
Sometimes there are more than one possible answer but you have to choose the most appropriate one. For that, you need to know which one you should use. You will need too much practice to have a good grip on this section. You can practice atleast 2 paragraphs per day daily. 

You should practise it for hard. Take previous year papers and see the questions and answers 
of Editing,Omission and Gap fillings . Practise  English grammar regularly .