Go into any gym with some serious lifters and you’re bound to see people using a weight belt, but to be honest most people have no idea how and why to use one properly.
The first step to using the weight belt correctly? Learning to Breathe and Brace. Filling your entire torso with air and learning to brace (stiffen and squeeze against the air) is the absolute first step of the entire process. Learning to breathe and brace for most people looking to get stronger is the fastest way to feel stronger in all of your lifts. Proper bracing also increases the safety of the lift making it less likely for injury.
So when do you use a belt? After you learn to breathe and brace and once the lifts get to a weight that challenges your ability to maintain that control. To use the belt, tighten it about bellybutton height to the point that 2 fingers can slip between you and the belt. When you’re ready to lift bracing against the belt will fill all of the available space and provide extra support for the taxing lift.