This is not difficult or precise. It just takes a few days.
Get a big jar. One that will have plenty of headroom after you’ve added the initial ingredients.
Add about 150 g and 150 ml/g of warm water. When you add flour and water, you’re always aiming for fairly equal weights.
Cover with a tea towel. The flour and water need to interact with the yeast in the air, but you want to keep undesirables out.
Every 24 hours—at around the same time each day—your starter will need to be fed: discard about 100 g of your starter then add about 50 g of flour and 50 g of water. Mix it well. If it starts to get a bit thick or thin, add a little more water or flour to keep it more like a thick pancake batter than a bread dough.
You should start to see bubbles after around 3–5 days. When it’s quite bubbly, discard all but about 100 g then add 200 g of flour and 200 g of water. 12 hours later you’ll essentially have a leaven which can be used in your first loaf of bread.
The remainder of your leaven is now your starter, which will need to be maintained.
Maintaining Your Starter
Continue to feed your starter (discard 100 g, add 50 g flour and 50 g water) for the next few days to make sure you’ve really built up a strong culture. You can then put it into maintenance mode.
If you plan on only baking once a week or less, store you starter in the fridge and make a leaven each time you bake.
The yeast is a group of hungry microorganisms that need to be fed flour. If you have a small amount of starter, a single feed goes a long way. If you have a large amount of starter, the same sized feed could leave them hungry. Storing your starter in the fridge slows the hunger. The amount of starter you discard and the size of the feed defines how sour it will get.
If you don’t bake but do want to make sure it stays alive, you can give your fridge-starter a small feed once a week. Although these infrequent feedings might help, I have left a starter in the fridge for months without feeding and it stayed alive.
Over time the starter will get more distinctively sour. To get a younger starter again, scrap all but a tablespoon and add 200 g flour and 200 g water. 12–24 hours later you’ll have a leaven to bake with, the leftovers of which can become your new starter.