How to start a sourdough bread starter
Begin with freshly milled flour (or as fresh as you can get). In a glass, ceramic, or steel container, combine one part water with one and a half parts whole wheat flour, mixing thoroughly. The start will be thick, not runny, and more like a paste than a liquid.
Cover the container with a plastic bag or plastic wrap, or cover it with a plate. Don't put an air-tight lid on it, or you might be in for a surprise. Place out of direct sunlight, at room temperature.
After 12 hours, check the start to see if it's begun to have bubbles in it. If not, stir well and recover and let sit for another 12 hours. If no activity is seen in the start after 48 hours, you may need to begin again.
When you see bubbles, and the start has risen a bit, then it's time to feed it by adding another part water and another one and a half parts flour, stir and cover again. When the starter has risen to twice its volume, discard half (in the compost, right?) and feed again using the same proportions of flour and water.
It's important to wait to feed the start until it has doubled its size, so you need to check it every 12 hours and either stir it or feed it. After the first five feedings, you can begin using white flour to feed it, which will help to build a strong culture.
Once your starter is active, it will be bubbly and smell yeasty, doubling its size after each feeding (which is why we discard half of it each time, or use it to bake with). After a week of activity, your sourdough starter is ready for use, and if you aren't going to bake right away, storing it in the refrigerator will slow down the culture.
The starter grows from bacteria and yeast already present in the flour, not usually from outside sources (though some are), so 'collecting' a culture from the wild isn't really necessary or desirable. The following sourdough starter method has worked well for me when I have neglected mine or moved and needed a new start. I learned all that I know about it from Sourdough Home and S. John Ross, who both have comprehensive sourdough resources available on their sites.