How can we help users to allow webcam access?

Assume a web application requires access to webcam and microphone. Browser compatibility has been checked and the required hardware is definitely present. When the app enters "chat mode," the user will be presented with a confirmation dialog that varies by browser:

All the user is left to do with is confirming the browser's security dialog.

However, it turns out that some not so tech-savvy users are not quite sure what do to at this point. It takes them at least a moment to figure out what to do next. If they are not regular users of web video chat applications, the dialog is not familiar to them at all, and they don't even expect it.

What best practice helps users understand why they need to confirm camera use at this point and how to do that?

Should this be explained before they get to the actual video chat ("On the next page, your browser will probably ask for confirmation that FooApp is allowed to use your webcam. In order to use FooApp, please confirm this.")?

Should the app show some kind of instructions while it's waiting for permissions, instead of just a blank screen ("FooApp is now ready. Please click 'OK' in the security confirmation dialog to share your webcam image.")?

This is also complicated by the fact that the dialog looks different in major browsers.