Create a web filter profile that blocks all sites except those you explicitly allow. In this example, users will be blocked from all sites except fortinet.com. You can do this by making a URL filter that has an entry that blocks all sites, and entries that allow individual sites. Ensure the entry that blocks all sites is the last entry in the URL filter list.
The original article/video can be found at FortiGate Cookbook – Blocking all web sites except those you specify using a whitelist.mov