Is to stir a goodly spoonful of a good joghurt into the milk and let sit for a a little over a day - in a spot where there is a consistent just slightly warm temperature.
Some people add a portion of Milk Powder to give it more body {another reason there is so many thickeners in store-bought}.
But if its whole unseparated milk, the milkfat also seems to give it a thickness unlike using non-fat milk.
YOu can buy Yoghurt-makers - but generally you get just a few cups for your time, but they at least are sealed units that keep it at the right temp.

You can also place a few spoons of some Acidophilus Culture from the HFS if you want to increase the enzymes in the product. Remember though DON'T add the capsules if it is capsule form dump those empty caps away.