Weigh out 4 ounces (113 grams) of all-purpose flour into a quart-sized container. If you don't have a kitchen scale, you can measure out 3/4 cup plus 2 tablespoons. Next, add an equal amount of water (4 fluid ounces / 113 grams / 1/2 cup). Vigorously stir the mixture together until it's very smooth, scraping down the sides of the container.
Cover the container with a paper towel or plastic wrap and use a rubber band to secure the top in place. If you'd prefer to use the actual top that comes with the container, leave it just slightly ajar or open so it's not airtight. Let the container sit at room temperature for 24 hours.
You may or may not see some bubbles starting to form on the second day. If you see them, that means the yeast is present. If not, don't worry, they'll get there. The starter should smell fresh, slightly sweet, and perhaps a bit yeasty.
Add an additional 4 ounces of flour and 4 ounces of water to the starter. Stir the mixture together until it's very smooth, scraping down the sides of the container.
Cover the container with a paper towel or plastic wrap and use a rubber band to secure the top in place. Let the container sit at room temperature for 24 hours.
By now, you're hopefully starting to see some activity. If not, don't worry and keep going. But be on the lookout for smaller and larger bubbles along with a slightly sour, yeasty smell.
Add an additional 4 ounces of flour and water, stir vigorously, cover, and set aside for 24 hours.
By now, you should be seeing some activity. The starter might have even doubled in size since you last fed it! (This is why I recommend a large jar). The starter might also be thinner in texture, which means it's hungry. It should have plenty of bubbles and smell very pungent and sour. You can try tasting it.
Weigh out the 4 ounces of flour and water, stirring them into the starter until smooth. Once again, cover the container and set aside for 24 hours.
Your starter should have doubled in size if it's active. If you look on the side of the jar, you'll be able to see the air bubbles which are giving the starter its rise. It's pretty cool.
By now, your starter should be ready to bake with. If you're still not seeing a lot of activity, don't worry. Keep up with the feedings and you'll get there in another day or two.
What you want to start doing now is discarding some of the starter instead of continuing to bulk it up. The "discard" is what's used to make bread while the rest is fed and saved for future baking.
Feeding Your Starter
If the starter is left at room temperature, you'll need to feed it every day. If you place it in the refrigerator, you can feed it once per week. There are also ways to dry it out for long term storage, but I'm not going to cover that here.
For daily or weekly feedings, measure out 4 ounces of starter, 4 ounces of flour, and 4 ounces of water. Stir the ingredients together vigorously until smooth, then cover and leave at room temperature or refrigerate. Discard the remaining starter or use it in a recipe.
I strongly recommend reading my entire post before following the recipe!