1 cup flour (I use spelt, you may use wheat, or rye, but anything else, I'm not really sure about. Kamut would probably work fine as well)
Use a large, clean jar (canning jars are perfect, but old mayo or pickle jars work great too!). Add water and flour and stir until well mixed. Put up on top of your fridge, or on an out of the way counter (just anywhere that's reasonably warm), and cover with a clean towel.
Every morning and every night, stir it around until it is well mixed again (it gets clumpy and it tends to separate a bit as well). At around 3-5 days, it will start to get a bit bubbly and smell sour (if it smells rancid, something has gone wrong, and you'll have to start over- to encourage you, I've done this many times over the years and have rarely had a batch that didn't work).
Store it in the fridge with a tight fitting lid.
Recipe by Keeper of the Home at https://keeperofthehome.org/a-lesson-in-making-sourdough-bread-for-yall/