Thank you for your valuable input..Start reading magazines and newspapers like nytimes, TIME, etc., They use complex language and words and you would be able to build your vocabulary gradually. If you need a cheat sheet, start preparing familiar words that you have known and look for synonyms online. For e.g, a simple word like 'boring' has the following alternate words. Same goes to all the words that you might be using.
synonyms: tedious, dull, monotonous, repetitive, unrelieved, unvaried, unimaginative, uneventful;
characterless, featureless, colorless, lifeless, insipid, uninteresting, unexciting, uninspiring, unstimulating, uninvolving;
unreadable, unwatchable;
jejune, flat, bland, dry, stale, tired, banal, lackluster, stodgy, vapid, monochrome, dreary, humdrum, mundane;
mind-numbing, wearisome, tiring, tiresome, irksome, trying, frustrating;
informaldeadly, ho-hum, dullsville, dull as dishwater, plain-vanilla
I will surely read them and try to use them in my writing and speaking
Appreciate that