Wash and sterilize a 1 quart mason or a plastic container. Container has to be non-reactive, so no aluminum or copper containers.
Add the cream to the jar, leaving about 1½ inches of space on the top. Add the yogurt and stir together.
Cover with a coffee filter, cheesecloth, tea towel, etc, and secure with a rubber band. Leave the container on the counter for 1-2 days.
Check each day to see if it has thickened. Should thicken in no more than 2 days, but normally in 1 day.
Once thickened, remove the coffee filter, cheesecloth, etc. and cover with the regular container lid and refrigerate for 1 day, then it is ready to use. Enjoy as you would use sour cream.