Pour the low-fat or skimmed milk into a small saucepan. Heat up over a medium heat until the milk begins to boil. Immediately bring the saucepan away from the heat and pour into a heatproof jar that has a lid.
Leave aside to cool slightly until the milk is just warm when tested on the wrist. Stir the milk to mix the layer of skin (that is visible on the surface) to combine with the lukewarm milk. Add in the plain yoghurt and cover the jar. Leave it aside, undisturbed, in a warm place or at room temperature for the yoghurt to set. This takes about 9-10 hours.
Once the yoghurt has set into a thick consistency (similar to custard), put it into the refrigerator. It will then firm up slightly when thoroughly chilled.
Homemade yoghurt keeps for up to 6 days. Add any chopped fruit if desired.