The point of the belt is so that prior to your repetition, you take a large breath of air into your belly and hold it. In order to protect and tighten up your core, you use that belly full of air to push hard against the belt. Use the belt anytime you'd like. There is no right and wrong. It will not weaken your low back as many say it will because you are using the belt to keep your lower back flexed and tight.

Take a look at a 10mm inzer lever belt. I have a 13mm and I powerlift. These are great belts.