First of all these aren't "fancy" words. These are words any 8th grader should know how to use. Words like 'therefore' and 'thus' are used in the middle of sentences normally after commas. They may be considered "fancier" versions of words like "but", "and", or "or".
Ex. "My eight page essay is due Sunday therefore meaning I need to write, at least, two pages a day.
some of it just takes repetition. try to start by not ending sentences with prepositions. learn a couple new words and find a way to use them. a few weeks later learn another and so on. you have to increase your vocabulary first. it will take practice though.