Can you tell me more? Do I put some in buttermilk(how much),
Buttermilk is Cultured Milk. That is milk that already has cultures in it. It is the classic Mesophilic Starter. Some dairies will actually list which cultures are present in their particular buttermilk.
Yogurt is another Cultured Milk product, although Yogurt is Cultured with Thermophilic bacteria (plural). Almost all Yogurts list which bacteria has been used.
Flora Danica is a Mesophilic Culture that contains Mesophilic bacteria (again plural).
So you would not need to add FD to buttermilk since it is already cultured, meaning it already has bacteria in it. In fact you can, and perhaps already have, used Buttermilk in place of FD.
I think John has a wiki on the Starter Cultures and the Bacteria, on the new site.
You add bacteria to cream to make products like Butter, Cream Cheese, Crème Fraîche, Sour Cream, Mascarpone and so on...
Or you add bacteria to milk in order to make whatever kind cheese it is you are making.
The bacteria can be added via the use of commercially available meso or thermo cultures, or you can use buttermilk for meso and yogurt for thermo.