Making a Sourdough starter
The good news is that this is as simple as mixing water and flour. The bad news is that like a Tamagotchi, the starter will need daily attention for the rest of its life! To commence, take a measure of the flour that you most commonly use and mix it with an equal weight of boiled water (to remove the chlorine, bacteria don't like chlorine), say 100g of each (since 100ml of water weighs 100g that makes life easy). Put the mixture into a suitable container (glass or plastic), cover with a cloth and leave it to stand. If you read the Microbiology of Sourdough you will know exactly what is happening now. All the microorganisms in the flour have come to life and started multiplying! A typical sample of bread flour will contain between 2 x 104 and 6 x 106 CFU/g (A CFU is a colony forming unit, the measure of viable microorganisms) so your 100g starter could contain up to 600 million microorganisms! So the various bacteria and yeasts start feeding on what is available, they can't metabolise starch so nothing much happens until the amylases in the flour start to break this down into easily digested sugars. Then a feeding frenzy starts, the bacteria metabolise the sugars into lactic acid and acetic acid lowering the pH of the starter and the yeast thrive in the acidic environment and metabolise sugars into alcohol and CO2. What happens now is that natural selection kicks in and only those microorganisms that can tolerate the acidic conditions will survive. As each new generation of bacteria and yeast is produced in 20 to 30 minutes, it shouldn't take long to establish a stable symbiotic culture, one that is optimised for your kitchen. How long it actually takes depends on a number of factors such as the ambient temperature and feeding regimen, reactions go more quickly at higher temperatures and bacteria and yeasts multiply faster at higher temperatures (although after a certain temperature is reached they will slow down and eventually die, so don't cook your starter!). In practise, after you have established your starter and begun daily feeding it should take between three and five days to 'get going'. You will know when it is active because after you feed it the starter will become foamy and double in volume, this is when it is ready to start leavening.
You do need to feed your starter of course. It doesn't take long for the flora in the starter to consume all of the nutrients that were present in the initial flour and water mix so you have to replenish them. Feeding every 24 hours is recommended and is probably easy to fit into your daily routine. You have two options, you can either tip out half of the starter and replace it with an equal amount of fresh flour and water mix, or you can keep what you have and add an equal volume of flour and water mix. Either way what you are doing is removing or diluting waste products and providing a fresh source of food. You should use the former method if you have enough starter to make the amount of bread that you require, and the latter if you plan to do a lot of baking. You may notice when you refresh your starter that there is a thin layer of clear liquid floating on the surface (sometimes referred to as hooch), this is the alcohol that is produced by the yeast, here it is an unwanted by-product of fermentation rather than the desired product that brewers want. By the same token, the CO2 that we want to make our bread rise is an unwanted by-product for them.
Once you have a healthy vigorous starter congratulations, you are ready to commence baking with it!