You are already doing that in the Build section of the website design area.
There is no preview page or button. What you are looking at IS the preview, just as a customer would see it from the outside. It will not be visible to the public until you publish it. If you have an existing site, and then make changes to it without publishing, no one will be able to see the changes. They will be looking at the old site until you "Publish" after you make the modifications. So what you put together in the Build section is what customers will see once it is published, but not until it is published. Only after you hit the orange PUBLISH button will the customers see the new work.
If you have not published a new site creation, you can look at it in the Build section, make changes until you have it the way you want, and then hit the PUBLISH button. If you see something needing changing after that, merely make the change, and hit the PUBLISH button again. They won't see it until you publish it.
