It's helpful to have a scale to measure out the ratios of starter to flour and water here, but if you don't have one, measuring cups or even just eyeballing will do in a pinch.
The rule of thumb is to discard part of the starter when you feed it because otherwise it'll just keep growing and growing and eventually fill your home and/or eat your block, which is not the result we're going for. It's helpful to have a scale to measure out the ratios of starter to flour and water here, but if you don't have one, measuring cups or even just eyeballing will do in a pinch.