This article is from the Bulgaria FAQ, by Dragomir R. Radev firstname.lastname@example.org with numerous contributions by others.
RECIPE 2 - Suggested by other netters and used regularly by me without
failure until today:
*) Boil the milk
*) Cool down to 60 C and pour into jars.
*) When around 50 C cover with towels.
*) When 41-44 C (warm but not very hot for your small finger), put
1 (heaped) tablespoon of yogurt (with live bacteria) per jar
(0.5-1 litre). Mix well using a fork.
*) Wrap with a woolen pullover and a woolen blanket and put in
a warm cupboard.
*) Leave for approx 8 hours.
*) Uncover. If OK, put in cold water and later in the fridge
(do not eat it straight away - it is not very tasty warm :-))
If for some reason not ready yet :
.) Put 1 or 2 more tablespoons of yogurt
.) Put the jar in 60 C warm water for 30-40 min
to warm it up.
.) Wrap it well again.
.) After 6 hours it will be OK (95% of the cases).