OK, go to the store and get:
Warm all your milk to room temperature or a bit higher - maybe 80 degrees or so. You want a good bacterial environment.
Stir in any flavorings you want - I like about 2 Tbsp. of Gilroy Crushed Garlic. Yumm yumm yumm. Also, some salt if you want to, but not too much as to kill the bacteria!
Stir in the buttermilk and let it sit and stew for about 8 hours. The little S. Mesophilus bugs will invade the milk and do their thing. That's what makes buttermilk taste like sour cream or cream cheese.
Next, dilute your rennet in water (maybe 1/4 cup) and stir it into your milk. If you're using veggie or Junket rennet, use about 4 times the recommended amounts. Once stirred, let it sit overnight - about 12 hours in any case. It should congeal into a curd that looks like very soft tofu.
Once the curd is set, pour the contents into a cotton gauze cloth in a colander. (single layer "birdseye" diapers - available at any dept. store or baby store are perfect) - don't use a cheesecloth for this or you'll lose all your curd - it's really loose and gloppy. Let the mixture drain for about another 12 hours or so in the sink (or over a bucket if you have pigs, they love fresh whey and it's good for them.) When it's drained, peel the cheese off the diaper and put it in a container. You have cream cheese.
You can salt it now and not be in danger of hurting the cultures, which have already done their job, or you can leave it unsalted, as I do, and salt it to taste when you use it. It's great on bagels and crackers.