Belting is essentially singing high producing a strong, beautifully powerful sound. It's important to breathe from your diaphragm and open your mouth wide while belting, and there are lots of exercises you can try that will help you strengthen your voice. If you belt improperly, it can cause damage to your voice and throat, so make sure you stop singing if you feel any sort of discomfort.
Stand up as straight as possible. You'll have a hard time belting properly if your body is slouching. Hold your head up and place your feet shoulder-width apart so that you're comfortable yet strongly vertically aligned.
You can keep your arms at your sides or wherever else they're comfortable.
Breathe from your diaphragm.
Release all of your tension by shaking out your body. Belting puts tension on your vocal cords, to it's important to counteract this by relaxing the rest of your body as much as possible. Relax your shoulders and pull them back so you're standing up straight. Shake out your arms and legs if necessary so that your body loses any tension.
You can also do some jumping jacks, stretch your arms up over your head as high as they'll go, or even try out some yoga to settle your muscles and mind.
Aim to stay relaxed before, during, and after singing.
Practising the Belting Technique
Keep your mouth open and your tongue down. The more you open your mouth, the better the sound you'll produce. Your voice needs to be able to fill up your entire mouth as it reverberates, and lowering your tongue so it's not in the way will help you produce a stronger sound as well.
Relaxing -- not forcing -- your tongue down helps control the increase in air pressure happening in your mouth.
Practice keeping your mouth open while singing and your tongue down until it becomes muscle memory.