Choose a belt type that fits your climbing style. Some belts are designed for multiday, multipitched routes, while others are specifically for competitions. If you are unsure of the type, you probably want a multipurpose belt.
Try different brands and styles. Every belt will fit differently, and it is important to find a style that fits the shape and cut of your body. Make sure to try the belts while wearing your climbing clothes or other similarly fit clothing.
Fit the waist belt. It should wrap just above your hipbones. Tighten it so it is snug but not uncomfortable and does not restrict breathing. You should be able to fit only two fingers in between your stomach and the belt. Make sure there is at least 2 inches of strap left over after the buckle is secured.
Fit the leg loops. These should also be snug but not uncomfortable, with at least 2 inches of leftover strap after the buckle is secured. Adjust the distance between the leg loops and hip belt to your comfort.
Check for comfort. Move around and make sure nothing chafes or irritates you. You want to be able to stay comfortable in the harness all day.