For example, in the squat, you breathe in and get tight at the top, hold that breath down and back up again, and then release your breath once you’ve locked out the rep at the top of the squat. In the deadlift, on the other hand, you take a breath in and get tight at the bottom of the lift (i.e., when the bar is on the floor), hold that breath all the way up to lockout and back down again, and then you release your breath after the bar is on the floor.
One last note - don’t try to push your abs or your belly outward into the belt. Again, don’t complicate this - take a breath, hold it, and get tight like you expect to get punched.
What NOT to Get
Don’t get a tapered belt, i.e., one that is wider in the back than it is in the front. The extra material in the back is just a waste of material and isn’t doing what you think it is.