OK so lets say.\n\nthey became teachers.\nThen how do you conjugate this?\n\nelles sont devenues les profs?\nor\nelles sont devenus les profs?
Elles sont devenues profs.\n("les profs" means "the professors", so the "les" isn't necessary).\n\nYou have to add the "e" to make it feminine, the "s" to make it plural, just like with all other Vandertrampp verbs.
Here's a tip.  Go to France on an exchange program.  You will learn more French in one month than in a year of study.  Check around.  There are some summer programs that are not that expensive.