Place the tablespoons of sour cream (or milk kefir) into the bottom of a quart jar.
Pour warmed cream* over the sour cream and fill the jar.
Stir thoroughly.
Add a loose fitting lid and let the cream sit on your counter for 12-48 hours.**
Check the cream after 12 hours to see how it tastes. If you like it, the cream is done culturing. See below for several notes.
Notes
*If you want your sour cream thick and firm (more like what you find in the store), heat your cream to 175 F and keep it there for a few minutes. Cool it down to at least 120 F.**The longer the cream sits, the more cultured, and therefore more flavorful, it will be. The cream will thicken as it cultures. If you want to speed up the process, you can insulate the jar in several layers of towels in an ice chest or an insulated bag. If you insulate the cream while it cultures, it should be done in about 12 hours. For the first 12 hours, this cream is more correctly called crème fraiche and is still slightly sweet. It becomes tart as it cultures longer, becoming more like the American version of sour cream.