I've already got a sourdough starter. But that's not very helpful if you don't. So I'm making a new starter from scratch. One way is just to keep a piece of yeasted dough aside from a baking and mix it with equal parts of water and flour. Over days it will sour. For any purists though here is the process from scratch:
Pour one cup of tap water (boiled and cooled to lukewarm) or bottled water (hence chlorine free) into a clean vessel (glass, ceramic or plastic) which has a lid.
Tip in one cup of rye flower:
Mix (to a porridgy consistency)
Cover with a muslin or paper towel, and leave somewhere out of harm's way in your kitchen
Wait several days.
The pictures above were taken today at 9 am so I'm going to have to wait and see if it works!