When you're creating an advert, selecting 'I'm happy with my ad' will save any changes you've made. You can continue to edit it within the 'vehicles you're selling' section of your account.
Your advert will stay as a draft until you've paid for it. After paying, your advert will go live.
Good to know: Your draft advert will last for 31 days from when you started it. After that time, it will be deleted and you'll need to start again.