Previously I was writing about additives they use in factory made bread. The potatoe is also can be considered an additive since it is used as a substitute to flour.
But I use it and I start the recipe with boiling some of it. I peel and slice them up first so it cooks faster. When they are ready I cool them down with cold water and/or leaving them to cool down by themselves.

I mix the flour and the potato. I squish the chunk of taters by hand. I keep about 2:1 ratio, I use cca 400 grams of flour to 200 grams of potato. Maybe a little less flour about 350 g. Really don't need to be exact here.
This mix to become dough also needs water. The sourdough will soften up more the stuff anyway, so at this point I add only a little. I keep it relatively dry, and make a dough that I can shape into a ball, and it won't stick to my hand much, or to anything much. So just a tiny a bit of water added gradually.
When the dough is done, before we would add the sourdough we should let it rest. At least half an hour but for the last one I forget about it for about 2 and half hours. This makes no harm to it.