Would anyone mind helping me convert this to sourdough?

There's a lot of confusion about "sourdough" bread.  It's actually strictly defined as a bread  prepared using the addition of dough left over from a previous batch.  To lend the "sourdough" technique to the recipe in your link, simply save a portion of the dough and set it aside to ferment.  Then add it to your next batch.

That said, if you want to prepare a sourdough with a flavor/texture that resemsembles the french batard/baguette/boule that is typically found in commercial bakeries, you'll need to find another recipe.  The recipe on your link is closer to a Brioche than anything else.

However, if you're bound and determined to try it, eliminate the milk, butter, sugar and eggs; add 10 - 11 ounces of warm water and have a go at it.  Frankly, unless I were feeding an army I'd cut the formula in half. That amount of flour makes a lot of bread.

"I want this exact one, just sourdough.."

And therein lies the difficulty.  When you convert it to sourdough, it will not be the exact one.  The flavor profile of the bread will change significantly when made with sourdough.

Take out 1/3 of the total flour and milk in the recipe and replace it with your sourdough starter.   Your starter should be made up of the same proportions of liquid and flour as in the recipe.  Treat this starter as your yeast and follow your recipe from there (minus 1/3 of the liquid and flour that is now coming from your starter).  The rising times will be lengthened significantly and everything about the dough will feel a bit different.  This is a starting point for what you want and will undoubtedly require some trial and error with adjustments along the way as you make this recipe again and again.

I am assuming that you have some sourdough experience.   If not, this a a very difficult place to start.  In that case I would recommend that you learn to bake a simple bread using sourdough.  Once you have basic sourdough skills, you could then return to this recipe.

Jeff

Converting sweet doughs from yeast to sourdough is an exquisitely difficult endeavor, and typically you still end up with some yeast in it.  And it most definitely will not be exactly the same.

The first thing I would try is just taking some of the flour and milk and making a preferment. I would take most of the milk and 1 c flour and add about 2 tbsp active starter. Let sit for 6-12 hours until fully ripened before using in the recipe.Simply subtract the flour/milk from the final quantitites of the original recipe. You may still need to add yeast to the final recipe initially untill you know how your starter will behave with this sweeter dough. I do this all the time to convert non-sourdough recipes. It sometimes takes a few times to get the proportions down and it can depend on how you keep your starter. I often use instant yeast (in small amounts)as well  as a starter to decrease production time-there are only so many hours in my day.

Starter: I estimate my starter is 125%-150% hydration and before I use it in a preferment, I feed it 2 or 3 times (either every 12 or every 24 hours depending on how long it has been since I last used it) to really activate it. If it has been a few weeks since I baked, I might need to get it back to health before I can use it for this purpose. An alternative is to add a pinch of commercieal yeast to the flour/water preferment. That works,too.

Or you can try a different recipe-just don't do the fruit and flavors:

http://www.thefreshloaf.com/node/26511/sourdough-panettone-2011finally-formula

I called this "Pannett-oche" becasue I based the dough on my brioche recipe. So don't do the fruit and flavorings but this has a sourdough base brioche recipe.

It is wonderful when you have a recipe that just works but when you start doing changes it really does behave differently and you have to experiemnt a little to get it to the "it just works" stage again. So-pull out your notebook and recipe and start baking delicious experiments.

keep closer to the original taste and flavor but want to use a naturally cultivated as opposed to commercial yeast, I would convert it to a Yeast Water recipe by replacing 60 g of water with yeast water and using 60 g of the flour to make the first build of a YW levain that weights  120 g.    Then 3 hours later I would add 60 g of flour and what ever water to equal the hydration of the finished dough and let it double.  The total liquids and flours and hydration remain the same that way, the commercial yeast goes away and the taste is nearly the same as the original.

I already hear the question and here it is answered:

http://www.thefreshloaf.com/node/30412/yeast-water

Good advice , if that is the direction you want to go.