Oh, dear - it's SO much harder to put these things on paper - or guess I should say "cyberpaper" - than to just show a person! But for all that . . . one way to use starter in place of the dry yeast in your recipe is to put your starter in a big bowl or other container (I use my ancient oval gray enamel roaster) and add all the liquid, all the sweetener, and enough of the flour called for in your recipe to make a batter that is somewhere just a bit west of muffin: a really gloppy batter that tries to hang onto your wooden spoon but can't quite. At this point your batter has become, offically (in some circles anyway!), a sponge. Cover your bowl, set it in a warm place and leave it until it gets bubbly and almost doubles in volume: usually one to three hours. I check mine after the first hour or so.
When your sponge is bubbly, nearly double, and looks active and feisty (sometimes you can actually HEAR it working, no kidding!!), stir it down with your wooden spoon. Scald out a jar that will hold twice as much starter as you began with. Half fill the jar with batter, set the jar lid on loosely, and put the jar in the refrigerator. This is your starter for next time. Check the jar after a couple of hours and stir it down; repeat the checking and stirring down until the batter convinces you it's going to hold still - this takes usually 3 stir-downs in all. I always let mine sit overnight and stir it down one last time before tightening the lid on the jar. I do not like jars springing surprises on me.
Now, back to the sponge! Add the salt, the fat, and the rest of the flour your recipe calls for, and proceed with your recipe as written. Your dough will usually rise much more quickly with this method owing to the head start it got from the sponge. And that's basically it.
A Few Notes:
If you want to double or triple your recipe, you don't need extra starter; it will just take longer for the sponge to double. If you're in a great tearing hurry, you can always give it a kick in the pants with some dry yeast, but it isn't necessary otherwise. Also, you can set your sponge--oops, I mean make your batter--the night before and put it in the fridge until you're ready to bake the next morning.
You can vary how fast your batter will rise and turn into a sponge by adjusting the amount of sweetener you mix into it at the beginning: less sweetener=slower rising; more sweetener=faster rising.
Oh, one more thing - I've never made a sponge using any liquid other than water. If my recipe calls for milk I use water for the sponge and then add powdered milk along with the flour etc when making the dough.
Sorry this is so long. The pictures in my head keep interfering with my digital communication! (See first sentence, 'waaaaaay back there!)
