Make certain of something/of doing something - Idioms by The Free Dictionary
make certain of something/of doing something
make certain of something
to check something in order to be sure. Please make certain of what you want to do. Would you please make certain of the number of things you want to order?
make certain of something/of doing something do something in order to be sure that something else will happen: You’ll have to leave soon to make certain of getting there on time.