In a glass jar – vigorously mix all ingredients with a wooden spoon to aerate. Adjust the water or flour amounts if your starter is too dry or too runny. The amounts don’t have to be exact. Keep covered at room temperature, with a tight mesh cloth, or paper towel with an elastic band to keep it in place. Leave at a constant temperature at 20 to 28°C. If your house is cool you can leave the starter in your oven with the light turned on.DO NOT TURN ON YOUR OVEN!Make sure fruit flies can not get through the cover.
In about 12 hours (breakfast and dinner time). Discard all but 40 g of starter and feed this with 40 g of the same flour you have been using. Plus 50 ml of filtered warm water.
Put the discard into its own jar to be kept in the fridge. You can make pizza, pancakes, crackers and even cakes with it later on.
Keep feeding 40 g of the starter on a regular schedule, twice daily, until it has grown in size by 30-50% and is bubbly, with a mild yeasty smell.
When you start to see a regular rise and fall of your starter with feeding, then you know it is ready for baking. When it peaks and has a nice dome it is ready. If the starter has fallen after peaking, by an hour or two. You can still mix it into a dough, but if you go much longer than that, you will not get a good rise from your bread.
At this stage you can make some sourdough bread, or keep the starter in the fridge until you want to bake bread.
Keep the starter out of direct sunlight in a warm place. Not too hot and not too cold.
Gluten free flours are expensive so keep the discard flour and always save some of the starter. After the starter is established and active, you do not need to use huge amounts of flour to feed it. Most recipes you only need 100 to 150 g of starter. Plus the 20 g to keep your starter going.