make certain of

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?
See also: certain, make, of

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.
See also: certain, make, of, something