Cookie I make a great yogurt every week and it is super super easy
Heat 2 L of unhomogonised (this makes a difference I find) milk to 80 deg for 20 mins. Let cool till 37 deg - in summer this takes 3 or 4 hours.
Ad 3 or 4 tablespoons natural yogurt and heat at 37 deg for 10 mins. While doing so , swush some boiling water around the thermoserver to sterilise and heat. Empty, pop the milk in, wrap with a towl and leave 8 hours.
I find that the first one can be runny, but once you re-sue this as a starter for the next batch it is thicker. I leave 1500ml as natural yogurt, strain the other 500ml for a few hours for greek yogurt or 24/48 for sweet or savoury labneh. the whey that comes off I use in cooking or to marinate meat.