Mix together the heavy cream with your milk kefir in a jar. The heavier the cream, the thicker your resulting sour cream will be. You can keep experimenting to get the consistency that you want with the cream you are using. If you want to end up with a thinner sour cream, you can also add in a little bit of milk to bring down the fat content.
Cover the jar, and leave alone for 24 hours.
Check on your kefir sour cream, and taste it. If you want it to be more sour, you can leave it out longer. If it's already at the consistency that you want, use it right away or store it in the fridge for later. (Storing it in the fridge will slow down the fermentation process and keep it from getting more sour as quickly).
Kefir sour cream will last a very long time, at least several weeks, if you don't eat it all first. ;) That said, it will likely get more and more sour as time goes by. (Mine never lasts long enough to find out!)