If it hasn't been in the fridge, you can use it straight away. If it has been in the fridge, take it out a few hours before making your sponge. Room temperature allows the natural leavens to multiply more readily in what you're about to feed it. The sponge should look like this when it's ready to use.
Basically the sponge recipe is:
1 1/2 cups starter or half your jar
1 cup unbleached bakers flour
1 cup water
Mix together in a glass or ceramic bowl, cover and leave at least 2-4 hours at room temperature.
As you can see by the photo above, I have two bowls out. One is for mixing my sponge (for bread) and the other is for feeding my starter (which gets stored away for later). The recipe is the same for both feeding the starter and making the sponge. I find it easier to do both things at the same time, as it ensures I always have the same amount of starter for making bread next time, and it's not so easy to forget quantities when you're doing it twice.
I use the larger bowl for the sponge. The smaller bowl is only needed to mix and feed the starter, as it gets returned to it's original jar afterwards. It's important to leave your starter jar out of the fridge for at least an hour after feeding, if you're planning to bake the next day. This gives the leavens time to feast before being slowed down again, by the cooler temperature.
You can choose to measure your starter out as the recipe indicates above (250mls or 1 cup) , but I found I had enough starter to just simply half the mix to make my sponge, and feed the other half which is the starter. Is it all a bit fiddley and confusing? Why not just jump straight into making a loaf of sourdough?
Well think of it like a body builder attempting to pump iron. In order to build muscle mass, you need to eat a lot of carbohydrate and protein first. You simply don't get the same bulk, if you stick to your regular diet. So it is with your sourdough bread too. You're about to ask it to beef up your loaf! Don't you want it to fuel-up properly first? The sponge is all about pumping leavens in order to get the bulk you want.
Once your sponge is ready however, you can now move on to making the bread.
More to follow...