Yes, there is a scientific explanation. Yes, this task has been performed for thousands of years, and by illiterate people. Still, the combination of simple ingredients, adding only air and time, to mix with flour, water, and salt - to amount to this sweet smelling, delicious concoction that is a bread. Well, it is easy to understand why ancient people created gods to which they attributed the force that natural yeast produces.
A pretty decent and everyday sort of miracle.
No lack of starter advice
No two people will give the same advice about how to create a starter. My advice, for what it's worth, is: Start two starters. If both thrive, wonderful, but probably one will. I would go with (1) a rye flour and (2) a white all-purpose flour. Some people use spelt starters. There is general starter advice from the Sourdough Companion forum. Many books and websites have sourdough starter suggestions. I especially like this all-about-starters post from the now-defunct Tartine Bread Experiment. Here is my advice. [Admire the lovely bubbles of fermentation in the photograph. Oohs and ahhs are appropriate.]
How to start a starter
Use a clean glass jar.
Use spring water - not tap water. Whatever chlorine or other additives are in tap water do not make for happy starters, particularly new ones. Go somewhere else for a scientific explanation, but that advice is universal.
Combining the flour and water: Add equal weights, but amounting to a quarter cup or more each of the flour and water. It does not matter whether you add a cup or half a cup of water. If you do not have a scale, then add a third of a cup of flour for every quarter cup of water. Mix well.
Cover the flour and water mix loosely with one of the following: a top of a jar, a paper towel or a loose piece of plastic wrap. The starter will need some air to get going and stay healthy. Keep the jar unrefirgerated and, preferably, in a warm kitchen. Don't sweat this; I started mine in a cool kitchen in the winter and it thrived.
Mix well a few times a day. Watch for bubbles. They might take a few hours or a couple of days to appear. When bubbles appear, throw away about half of the mix and feed the starter with about equal weights of whatever flour you are using and water. At this point, the objective is to get the starter going and not to produce a huge amount. Feed twice a day after bubbles appear.
Mental health warning
Do not be alarmed - you will be obsessed about this and constantly checking to examine bubble activity.
Consistency is critical. A starter should be more of a muffin batter consistency than a pancake batter. A wet mass, but not runny. It should not pour easily. Think thick and viscous. I typically add about double the weight of flour to water. Many people go for a 1:1 ratio. There is no wrong answer; whatever works is fine.
After about a week, if there is a bubbly, happy mass, congratulations! The first time or two using the starter, feel free to mix in a tiny amount (a quarter to a half of a teaspoon) of commercial yeast. Once the starter is healthy for a couple of weeks, it can go it alone to produce some fine breads. [Photograph of my pretty starter right before use and feeding. Notice the bubbles and the small amount.]
Sustaining a starter
Let go. Forget amounts. Forget fractions, as in use or throw away half of the starter before feeding it. Forget everything but the look and smell of a good starter. Add to the starter, use some, and/or cast away some.
Determine whether you are maintaining the starter amount or growing it. That will determine whether to keep or get rid of some when feeding it.
Adjusting with different flours
For making breads with different types of flours - whether spelt, whole wheat, rye or whatever - one can feed the starter with a different type of flour once or twice in preparation for making a bread with a particular type of flour. Or, switch the flour type occasionally just to play or inject a little different taste in one of your usual breads.
In the refrigerator - feed twice a week. Many experts recommend once a week, and that can be done, but mine looks weary by then, greyish water on top. Twice a week and it will never appear sick.
Out on the kitchen counter - feed twice a day. Once a day will sometimes do; sometimes that's a bit of a stretch. Twice a day guarantees consistent health of the starter - especially in the summer.
Change the jar about once a month. No one wants to see dirt or unwelcome mold. Sorry to mention that.
Some say starters are ready to use right out of the fridge; others say they need to be tended for up to three days. I take the middle ground. I usually take out the starter 24 hours, but sometimes 12 hours, before using it. I like to feed it twice, ideally, before putting it to work. On the counter, with some warmth, the sourdough culture thrives, becomes happy (as we all do with some warm air) and it is ready for action.
How much starter to use? Recipe instructions are not mandatory. More starter = less time needed for bulk fermentation of dough. Higher temperature in the kitchen has the same effect. Futzing with starter amount and room temperature can speed up or slow down the process. Personally, I like a nice long fermentation period so that those good bacteria can grow and produce a nice tangy taste in the bread.
Adjusting yeast or starter amounts
Many recipes call for commercial yeast. First, the amount given is typically geared toward quick rising - the more yeast, the quicker the rise. Generally, such recipes will include a tablespoon (or equivalent) per loaf of bread. Even without starters, one can improve the bread by using much less yeast - even as little as 1/4 teaspoon - while adding a great deal of time - generally going from a two-hour first rise to an overnight or longer first rise. I like the additional time because it enables overnight and all-day (go to the office) rises.
The rule is the same for starters: The more starter, the quicker the rise. Many recipes that include starters also call for dough made in stages with a preferment of biga, sponge, levain, poolish, or whatever variation. Again, I am learning to put in less starter for a longer time to rise, whether or not that includes a preferment or just an all-the-ingredients-together first rise.
Another variable in the starter or yeast equation is the temperature of the room in which the dough is rising. Rising times vary considerably from a 55-degree kitchen on a winter night and a mid-day summer kitchen of 90 degrees (for people like me who hardly turn on the air conditioning). In summer, I am learning to use a quarter of the yeast or starter I would add in the winter so that I can keep the rising time the same throughout the year.
Substituting starter for yeast
A common question is how to substitute starter for commercial yeast. Some use a rough general rule of a quarter to half cup of starter. That's me. Some meticulously account for the amount of flour and water in the starter as substitutes for amounts in the recipe, also adjusting for a preliminary build/preferment stage. I've done this as well. I really cannot say that the more mathematically-correct method works better. The loosey-goosey method ends up just as well.
There is good information on some freshloafonline forum discussions. I'm sure there are more and the forum is searchable. Here are some freshloaf discussions with a variety of suggestions:
Amounts of starter to use
Substituting starter for yeast
Going away on vacation? No need for a kennel
Leaving for more than a long weekend and there's no friend able, willing and knowledgable to feed the starter? No need to worry. Put the starter jar in a plastic bag and place it in the freezer. Defrost when you return and keep it out on the counter for a few days, if not a week, feeding it twice a day. The starter will take up to a week to forgive you, but it will revive, just not automatically or instantly. After tender love and care for those days, the starter will be ready for baking and to be put again in the fridge.
Trouble with a starter?
I found this all-about-starters post from the now-defunct Tartine Bread Experiment to be the most detailed and helpful information about troubleshooting starter issues and, indeed, about beginning one's own starter. Basically, in my opinion one should feed frequently enough, keep things clean, use filtered water, and stick to a starter consistency (whether like pancake batter, cookies or runny) that works for you.