I normally maintain my sourdough starter at 100% hydration. That is, every feeding, and therefore the starter itself, consists of equal parts of flour and water, by weight. It has a batter-like consistency and is therefore a “liquid” starter.
But not every sourdough recipe calls for liquid starter; some call for a stiffer starter, often at 50% hydration (that is, the ratio of flour to water is 2:1). It’s easy to take a portion of liquid starter and convert it to a stiff one.
The easiest way to do this is to start with a small amount of liquid starter, say 10g, and feed it with 20 g of flour and 10 g of water. Then at every subsequent feeding, continue to feed with a flour:water ratio of 2:1.
The absolute amounts depend on feeding frequency, temperature, and individual starter characteristics. After a feeding, it should be able to at least double itself in several hours and hold there without collapsing until the next feeding. During cool winter months, I find feeding every 12 hours at a ratio of 1:2:1 (starter:flour:water by weight) works well. In the summer it might be, say, 1:3:1.5 or 1:4:2. But flour:water is always 2:1.
After a few feedings the new stiff starter is ready to go. Use it at its maximum volume, a few hours after it’s been fed.