You can do this in one of three ways. The first is to obtain an established starter from a friend, feed it, and use it for baking. Another method is to purchase a dehydrated starter from a culture starter company and follow the directions for rehydrating, feeding, and baking with the starter.
If you're more of a do-it-yourselfer like me, then you may want to make your own. Honestly, you won't believe how easy this is. All you need is flour, water, a half-gallon jar, and a wooden spoon. Here's how I do it:
Day 1: Combine 1 cup wheat flour with 3/4 cup warm water. Cover with towel or coffee filter and secure with rubber band or canning ring. Place in a warm place.
Day 2: Add 1 cup freshly ground wheat flour and 3/4 cup warm water. Ditto above.
Days 3-5: Repeat.
The time it takes the starter to become active will probably depend on what yeast beasts you have hanging around in your home. To maintain your starter simply feed it every day and keep in a warm place, or refrigerate and feed just once per week. I also like to pour the starter into a bowl once a week and beat some air into it with a wooden spoon.
Make Your Sponge The Night Before:
I have found that giving the yeast time to activate without added ingredients results in a better bread. For this step you will need a large, non-reactive bowl, a wooden spoon, and the following ingredients:
Combine all ingredients with 3.5 cups warm water and beat well to incorporate air. It should be the consistency of pancake batter. Cover with a damp towel and place in a warm place, like next to a heater or in a warm oven.
Prepare Dough to Rise & Bake:
In the morning, get out your starter which should look active with bubbles and a bit of rising action. Give it a good stir and then remove 1 cup to set aside for your next batch of bread. To this you will add:
Add the butter/coconut oil to starter and fold in along with sea salt. Now add the flour one cup at a time, while mixing, until the dough pulls away from the sides of the bowl, but is still a little sticky. Remove dough and place on a well-floured surface. Now knead the dough for about 10 minutes or until smooth, adding more flour to work surface as needed.
Butter two bread pans and set aside. Now cut your dough in half and form two loaves. Place loaves in bread pans, give them a few diagonal slits with a sharp knife, cover with a damp towel (tea towel, not terrycloth), and place in a warm spot. This time of year I place them inside of my oven and turn it on warm every time I walk past.
At this point you wait until the loaves have risen past the top of the pan. Depending on how active your starter is this could take a couple hours or you may be baking bread around dinner time. This is something you have to make peace with when you bake with sourdough bread.
Once the loaves have risen to your liking you will bake them in a 350 degree oven for 45 minutes to an hour, or until the bottom of the loaf sounds hollow when you thump it. Remove loaves from pans and place on a cooling rack to cool completely before slicing.