It has been quite a while since I made sourdough bread. The last time I was making it my wife made me throw out the culture because she said it stunk. The other thing that has killed my previous starters was infestation by gnats. Yuck.
I've had 3 good starters in my life. The first was when I lived in Iowa City. I started that one using commercial yeast. It lasted a couple years and then I moved and threw it out. The next starter I had was in Chicago. I got it from this guy. He has a culture that dates back to the gold rush. It was an extremely tasty starter but it eventually got infested by gnats.
The last time I had a starter was in Kansas City. It was a wild yeasted starter (see below). It made the best bread of the three. I think I went on vacation and forgot it on the counter. When I came back it was moldy. Lesson learned - if you aren't going to use your starter for a long time, be sure to freeze it. Don't let it sit at room temperature unless you are actually about to bake bread. Instructions for freezing are included below.
This is a good time of year to start a culture. You can have a nice warm room temperature without too much fuss. I have used Peter Reinhart's system before and it made great bread. You can read about it in my favorite bread book The Bread Baker's Apprentice.
To make a sourdough starter you just need a little time, water, a little rye flour and some unbleached bread flour. You'll also need some sort of container to keep the culture in.
The culture is where the yeast and the lactobacillus for your bread comes from. You don't need any yeast to start a culture. Wild yeast is growing everywhere in the environment. It is especially prominent in rye flour and I suspect that is why Mr. Reinhart uses rye flour in the first step.
Combine 1 cup rye flour and 3/4 cup water in a bowl. Stir real well and cover. Let it sit on the counter for 24 hours.
Nothing much will have changed in your culture overnight. Add 1 cup unbleached flour and 1/2 cup water to the bowl. Stir, cover, and let it sit on the counter again overnight.
On day three there will probably be some signs of fermentation like bubbling and a slight rise in the dough. Dump out half of the starter. Add 1 cup unbleached flour and 1/2 cup water. Stir well, cover and let sit on the counter for 24 hours.
The dough should have doubled in size by now. If it hasn't, let it sit on the counter for another 12 to 24 hours. Otherwise, dump out half of the culture and add 1 cup unbleached flour and 1/2 cup water. Cover and fement until at least double in size. It may take from 4 to 24 hours.
At this point you can turn your sourdough culture into a barm or mother starter.
Take 1 cup of the seed culture from above and mix it with 16 oz (2 cups) water and 3 1/2 cups (16 oz) unbleached bread flour. Transfer the mixture to a clean container at least twice the size of the mixture. Cover with plastic wrap and let sit on the counter for about 6 hours until it is bubbly. Put the barm in the fridge overnight. It will be ready to use the next day and remain potent for 3 days.
Refreshing your Barm
You should refresh your barm the day before you plan to use it. Throw out or use half of the barm. You want to double the amount of stuff in the container by adding flour and water. For instance if there is 1 pound of barm left in your container, you refresh it by adding 1 3/4 cups (8 oz) flour and 1 cup (8 oz) water. Basically you need to add equal weights of flour and water to double the barm. Cover the barm and let it sit on the counter for 4 to 6 hours until bubbly. Cover and put in the fridge overnight.
Storing your Barm Long Term
If you are not planning on making any bread for a while you can put the barm in a ziplock bag which has been misted on the inside with spray oil. Put the barm in the freezer. You will have to take it out of the freezer several days in advance of any planned breadmaking. Thaw it out in the refrigerator. Once it is thawed you will have to dump out all but 1/2 cup of the barm and refresh it a couple times as described above.