The feature is pretty straightforward as all you’d have to do is launch Google Now by saying “OK Google” followed by the command which is “send a hangouts message” or “send a chat message”. After which another interface will popup in which you can compose your message. Alternatively you could just say everything from the start, like “send a hangouts message to <name> saying <message>” as this would save you some time.
It is unclear as to when this feature got activated but it appears to be live now, so if you’ve always wanted to be able to compose and send messages to Hangouts using your voice, you should probably go take the feature out for a spin. It should be noted that this feature is currently limited to Android handsets, so if you have an Android Wear device, we suppose you’ll have to wait for Google to bring that feature to Android Wear which hopefully won’t be too far off from happening.