All you need, some coarse rye flour and water in a cup. Ecological if you can get it, I am however trying with standard coardr rye flour here.

Day 1: Put in 2 tablespoons of water and one with flour. I use tap water here in Stockholm. If you have doubtful water in your tap, with chloride added, use bottled water or this step will not work.

Mix it up and let rest under loose lid (not air tight that is) for 2 days.

Leave it to rest in a warm area within your habitat (or cool area depending on where you live). Make it some 25 degrees Celsius if you can, no draft. Stir it a little once or twice a day.

One day has gone by. A little stir, I believe I see some small bubbles. And there is a slight sour smell emerging. It's alive and picking up an appetite for rye! So there is hope for this starter..

On the second day. Clearly it has come alive. So add 2 spoons of rye flour and equal amount water. Stir and be proud that the result is coming.

I realised that this is a good way to provide draft free but airy environment. Skip the lid. Let it enjoy its latest meal for another day.

On the third day. You have succeeded if the mix looks something like this. See those bubbles? It's ready to be used as a starter. See my separate suggested guide on how to use it in sourdough bread.

If you prefer you can make a whiter sourdough stater as well for whiter breads. I then replace coarse rye flour with a mix of 60% wheat flour and 40% sifted, i.e. fine, rye flour. Great for pizza.

If not baking today, put a cover on and store it in the fridge till needed later. Feed it once a week with 1 tablespoon of flour and equal volume water. That way you can store it forever. Good luck.