To master any language practice is must. To draft better letter, you need to have good stock of word power.
Further, you can do following:
1. Read English Newspaper daily. 2. Read incoming and outgoing correspondences in your office regularly. 3. If you have a patience, buy a book on letter drafting and read & practice. 4. If not, contact the undersigned, I will teach you letter drafting by email or through messenger.