Practice Precis and paragraph writing everyday.Paragraph writing is very important as it helps in essay writing.Solve past papers.
If possible try to solve one past paper in one sitting everyday.
Practice direct Indirect from past papers, various grammar books and Direct Indirect Made Easy.
Idioms are difficult to prepare and cant be prepared from a single book.Conventional idioms are not asked.Idioms already given in the past papers are hardly repeated.Prepare idioms and phrases from the newspaper.While reading newspaper,underline phrases and idioms and look for their meanings.Following sites can be used to find the meanings....
Answer.Com.
The Free Dictionary.
Cambridge Dictionary.
Merriam Webster