Hello and welcome @The_lady_in_pink!
This is one of my favourite positions and I’m also a very curvy gal! I’d say the main thing is about just completely and utterly owning it. This is your position, do what feels good for you and be the powerful person you are 
Some more practical tips:
- As @brufton suggested, blindfolds are an excellent way of working out the rhythm without worrying what you look like and any faces you may pull! When you feel a bit more confident with it, you can lose the blindfold and show him what he’s got.
- I find I can be my true wild self, particularly on top, when I’m wearing some good lingerie. If you feel confident and sexy in yourself then that 100% comes across.
- There’s numerous ways that you can do “girl on top” and so you have to find what works for you. For example, you can be leaning forward, sitting, leaning back, reverse, grinding motion, up and down motion, feet flat/squatting. There’s loads of options and you’ll soon find a few faves and work out which are most comfortable for you.
- Hold onto something to help your balance. My favourite is to lean over a partner and put my hands on top of my headboard for grip - this also gives them mucho boobs in face (and can help distract from other areas you may be less keen on). Another one is locking hands with a partner in the air and pushing against them if that makes sense? Sorry, very hard to describe! It gives you a sort of lever and more power I find!
- My advice with any of the motions/rhythms is to start off slow and then gradually get faster depending what feels good. This will make it easier to find a rhythm. Also, don’t feel like you have to stick to the same rhythm throughout, mix it up! (this will also help with stamina - and remember to breathe!)
- Practice without him. @For_Your_Eyes_Only_x made a really good suggestion with the duvet, I have done this numerous times! Also practice just moving your hips without moving the rest of your body if that makes sense?
Whilst I’m saying all that, when you’re actually doing it try not to focus too much about exactly what you’re doing and what rhythm you’re going at as this will distract you and throw you off! Just do what feels good for you and let your hips do the work 
Your partner knows your body and undoubtedly finds you super hot so OWN it 
A little note to add that if you really don’t feel comfortable doing this position then you absolutely don’t have to, there are so many other positions that you can try 