Originally Posted by A Zeee

For improving grammar, use any recommended grammar book. Understand rules and practice given exercises. For good writing expression one must have a decent vocabulary. You can prepare GRE high frequency list or Manhattan 501 vocabulary or both. One can also make their expression interesting and eye catching by using idioms. You need to prepare a decent list of idioms as well.
Develop a reading habit. Try to write summary of what you have read according to grammatical rules and by using whatever vocabulary you have. Get it checked from competent person. It can be any book, article etc.
Slowly your expression will improve. Try to read, listen, talk and write in English wherever possible. It will help a lot. At starting stage, you may make a lot of mistakes but this stage will end soon. Practice will make you perfect.