I know there are probably a couple of ways to make buttermilk, but I will post the way I make it here. I got my starter from Cultures For Health and made the starter mixture. From that first batch, many new batches have come out of it.
I take the buttermilk that I made and from that I take 1/4 of the batch and add enough (preferably whole) milk to make a cup. I put that combination in a small mason jar, and cover it with cloth or a napkin, then let it sit on the counter for about 18-24 hours. When the mixture is thick, I know I can put a lid on it and put it in the fridge. Sometimes, I put it in the freezer and take it out when I want to make another batch. If I want to make a recipe, and it calls for two cups of buttermilk, I have to make a new batch using 1/2 cup of the buttermilk mixture and add enough milk to make two cups to it.
If you have a recipe that calls for buttermilk, but you don't have buttermilk in the house, you can substitute. Take a TBS. of lemon juice or vinegar and place it on the bottom of the measuring cup. Add enough milk to make one cup. I use a little more lemon juice than that and I don't think it will ruin it if you don't have the exact measurements. Just be careful if you are making a recipe that calls for adding baking soda to the mixture. Don't do what I did and put the whole mixture in a small Magic Bullet cup! You will end up with a mess!
I found another page that tells about making buttermilk. The url is: http://www.livingonadime.com/homemade-buttermilk/
I haven't been to this web site before so I can't form an opinion on it other than it being a page that tells about buttermilk. The Cultures For Health one has a lot of recipes for yogurts and cultured foods, and starter mixes.