Sourdough starters are very flexible pets. Although they do need to be fed, it is up to you to determine how much and how often to feed it. For your own benefit, it is helpful to keep your starter on a consistent feeding schedule so that you can make consistent bread. However, if you happen to miss a feeding, or change the feeding schedule, you can often make the same breads with only minor changes to the formula.
To feed a starter, all you have to do is mix in some flour (to eat) and water (to keep the same consistency). The amounts that you feed, and how often you feed is up to you. When I first had my starter, I would just add a handful of flour, and enough water to get it back to my original consistency.
To express how you are feeding your starter, you can write the proportions as [starter weight]:[flour weight]:[water weight]. For example, if your starter weighed 100g, and you fed it 50g flour, and 50g water, you would be giving it a 100:50:50 feeding, or 2:1:1. This particular feeding schedule, which is very common, would double the starter weight every feeding, and keep it at 100% hydration.
You can keep your starter in a fairly large range of hydration levels. 100% hydration is fairly standard, and is a somewhat liquidy mixture. As you lower the hydration level, the starter begins to stiffen up, and turn more into the consistency of dough. If you decide to increase the hydration level of your starter, it will be able to more effectively eat the food (flour), so will become more active in less time. This also means you should be feeding it either a higher dosage of flour, or more often.
To have a very active starter available throughout the day, some professional bakers feed their starter twice a day. Others feed their starter once a day. You can get away with feeding your starter once every 5 days, but by then the starter will begin to look like it is almost dead (if using a 2:1:1 feeding). If you are feeding your starter once every few days, it might be helpful to give it a higher proportion of flour and water in each feeding (like maybe 1:1:1, or even 1:2:2).
If you decide to go on vacation, or don’t want to bake bread for a while, or just get tired of feeding on a consistent basis, starters do just fine in the fridge. In the fridge, they become much less active, and take a long time to go through all their food. If your starter was fairly active before going into the fridge, and comes out within a reasonable time period (perhaps two weeks), you put it to use immediately. They can survive for months in the fridge, although the longer you keep it in there, the less active it will be upon bringing it out. Also, the starter will continue to get more sour the longer you keep it in the fridge. After a while (a few weeks to a month), it will begin to develop a greyish liquid; this is excess alcohol, and can be stirred back in or poured off.
One thing you will always have to keep in mind is how much bread you plan on making compared to the feeding schedule you give your starter. If you are doing a 2:1:1 twice a day feeding, your starter will quadruple in size every day unless you are making bread from it. If you want to make a lot of bread for an upcoming event, but your starter is very small, you might want to feed it more often, or a higher proportion.
Make sure you keep your starter in a container with some extra space in it (don’t fill past 75% of the container). As the starter eats, it produces gas, and can grow in size. If you are monitoring your starter, you can often deflate it simply by stirring it or tapping the container, but if your container is too full it will explode out of it.
If you accidentally feed your starter something other than flour, it will usually be ok. Once I fed Blarf masa, and didn’t figure out what to do until the next morning. To get the starter healthy again, all I did was feed Blarf a large dosage of flour (2:3:3). On vacation I once we were all drinking rum, so I gave some to Dulce, and she seemed just fine.
When using your starter in formulas, make sure you keep track of what its feeding proportions were and how long since the last feeding. For example, in one of my most common breads, I use a 4hr 2:1:1 Blarf. To help you keep track of this, it is helpful to print out and use a Blarf Feeding Schedule.
Whatever your system is for feeding your starter, make sure it is easy for you. Your starter will be the basis of all your breads, so it is important that its upkeep is as simple as possible.