make certain of (something)
To check something in order to confirm that it is true, correct, or is happening. I just want call the restaurant and make certain of our reservations for tonight. I always get to the airport two hours early to be certain of making my flight.
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.