Essay: fundamental principle
Fundamental principles remain same : spend the first few minutes on brainstorming the idea.
Then rank those ideas in ordered way, start writing essay:
introduction >> body (+/- pros, cons) >> conclusion.
Book for your fodder points mention at last of the article.
RBI asks vocabulary questions directly from the passage meant for comprehension.
Letter / Memo / Report Writing
Read English newspaper on daily basis, particularly the economy section, letters to editor and columns to get additional ideas/fodder points.
practice yourself @home writing a letter to a newspaper editor and writing a news report.