For basic (and simple) usage, use only the "Accept" button. If the visitor clicks it, the cookie bar will slide out of view. The downside is that visitors who don't want their privacy invaded will leave the site.
Adding the "Policy" button - linked to a webpage with info about what data is being collected - will allow cautious visitors to be informed and then, hopefully, continue to browse the site
The "Decline" button should only be used if code has been added to disable ALL code - internal and external - which collects visitor data.
As mentioned above, this applies to analytics, YouTube movies, social media plugins and so on …