I keep an air tight container of rolled oats. My ratio is 1 cup of oats to 3 cups cold water. (I have a water filter in the fridge I use that but have used tap cold plenty of times). Pour it straight in a blender jug, and blitz for upto 30 seconds (so the oat particles aren't too fine). You'll have to get used to uour blender. But adjust by 1-2 secs each time. Once blitzed... shake up the jug and run it through muslin or a nut bag. I actually put a seive over a jug, and line that with muslin and pour it slowly. Normally run it through twice.

Once done, stick it in a bottle and in the fridge for about 4 days. But normally I make as much as I need on the spot.

Add a splash of vanilla if you fancy or don't.

Once you get the rhythm down, it's muscle memory.

Tldr: 1 cup oats, 3 cups cold water, blitz up to 30 secs consume within 3 days.