Start at "Alexa.amazon.com"... go to devices (in the menu on the left) and scroll to bottom of the page that comes up, and click on "Discover devices" (lower right side usually). Once the device is discovered, you can tell alexa to "pair" with the specific device it has to choose, by the name on the discovered list. It may be as simple as saying "Alexa, Pair to Blue Tooth Hub"... It may require the brand name such as"Alexa, pair to Samsung bluetooth hub"... You'll get used to the specifics as you go, but this should get you started
As an aside, you will also always be able to discover devices by saying "Alexa" discover devices", however, she will only tell you how many devices she discovered, and does not tell you WHAT she discovered, otherwise I would have suggested using that approach. Once you are very familiar with what is already there, you can tell if she adds any new devices or not, by how many she says she discovered after the "20 seconds" goes by while she discovers them.
Also, you can say "Alexa, discover devices" and she will and give you how many she discovered, thn you can go to the devices screen described above and it will show what she discovered as well... Always a million ways to get results with Alexa. Smart Phone, Computer, Tablet, and of course, Alexa devices.