A Sponge preferment is a mixture of flour, water and yeast that is left to develop for a period of time before being added to the rest of the ingredients for a dough.
There doesn’t seem to be any hard and fast definition of a sponge, it is almost a generic term for any preferment that is mixed, left to develop and then used. Some books discuss it with poolish and biga, some give it its own section but describe it in terms that make it sound like a poolish or biga.
Here’s one way to try it
Consult what your recipe needs.
Mix some of the flour, all the yeast and all the water.
Cover the container and let it develop for 2 to 3 hours.
Then mix in the rest of the ingredients and make the dough.
As you can tell, you can make a preferment of just about any proportions and use it and it will probably qualify as a sponge. I suggest you experiment a bit to see what proportions work for you. Here are some simple guidelines to get you started.
If you want the preferment to develop for a long time, make a preferment that is fairly low in hydration, something around 60%.
If you want to make a preferment that will be ready to use in a short time, go for a higher hydration, 100% or more.
A classic sponge, or so I am lead to believe, is as I specified at the beginning and is allowed to develop for 2-3 hours.