We can’t really help you with the centering specifics unless we can see your code. But here is a nice tutorial on centering that should hopefully answer any questions.
As far as submitting your project, are you using codepen? Some other public code repository? Developing on your own computer? We need to know where you are creating your project in order to tell you how to submit it. But in general, you need to be able to provide a URL to the finished project, which is what you will put in the Solution Link box.