Place the oats in a container with a lid, add the water and apple cider vinegar, stir well and leave to soak overnight. You can leave to soak at room temp or in the fridge.
The following morning, drain the oats in a sieve and rinse under a running water.
Add the oats to a pan, cover with milk, add a lid and simmer gently for about 5 mins. Turn off the heat and leave to continue soaking in the hot milk for a further 5 mins. This will help to really cook the oats through but prevent over drying or burning.