Boil the milk. Leave it to cool a little - you need to be able to dip and keep your pinky in it for 15 seconds.
Stir the yoghurt with the yeast and salt. Add it to the milk and stir well. Cover with a blanket or several layers of towels.
Leave it to sit for about 7 hours. Pour it into a cheesecloth and leave it to drain for about 12 hours (it may take less time).