Lots of good advice in here, so I won’t repeat most of it.

One thing that I didn’t see anyone cover is how to tell a culture is “healthy”. My recommendation for this is to use a container that you can add graduation marks to. Put some lines on it to indicate volume. Then when you feed your culture, keep track of what volume you have in flower/water. Then watch the starter volume over time. People often say: feed every x_time. In my experience it’s far more important to time the feedings based on the activity. Once you do this for a short while in your specific environment, you will get a feel for how often you need to feed. My house “room temp” varies quite wildly across my seasons. So I have to feed more in the summertime, and less in the winter.

Your mileage may vary.