The directions (with my comments/questions in italics) are :  (and my comments are in bold - Mini)

1. In a mixing bowl, combine all ingredients (except the optional yeast). Mix until combined. Knead dough by hand for 15 minutes or with a dough hook for 10 minutes (or until gluten develops). I take this to mean until the dough can pass the window pane test.  YES, that's what they mean.  I tend to do it differently.  I mix up a shaggy dough until all the flour is moistened and then cover and let sit 30 minutes to return to the dough and continue.  Saves me a lot of work while the dough develops gluten all by itself.

2. Flatten dough, place in a large plastic bowl. Dust the dough with flour and cover with plastic wrap. Rest the dough for 6-8 hours. If using yeast add to dough at this stage. Be sure yeast is well distributed throughout the dough . so, does this mean to add the yeast AFTER the 6-8 hours, YES , sounds like if the sourdough starter is too weak in 8 hours, and you're in a hurry, add some instant yeast to give the dough a boost, then the dough should be handled like a faster yeast bread from this point on with shorter rising times.  The sourdough then becomes more of a flavoring, using the 6-8 hours as preferment time.  

3. Knead dough onto floured surface until gluten development. (does this mean it LOST the gluten development while resting the 6-8 hours?) Knead until dough is smooth, shiny, and satiny (my dough is always smooth shiny and satiny after the first round of kneadingit looses this texture when I start to knead again, and becomes very lumpy and ugly). Test for proper kneading by pressing your finger into the dough, if the dough stays indented it is developed. (mine will stay indented, even though it is not smooth, and is lumpy)  Really don't understand this direction. Kneading a sourdough is too much action for a sourdough, I would gently degas the dough and do a rotation of envelope folds.  Shape into a round, rest for 15 min and then shape for loaves.  If kneading makes the dough lumpy, then don't do it.  If the dough is soft, folding during the bulk rise will do more for the dough than a long bulk rise of the dough just lying there.  Sourdoughs get wetter as they ferment, more so than instant yeast doughs.  They tend to relax more and therefore need help in maintaining their shape.   I tend to think of yeast doughs with clear lines between bulk rise and final rise, but with sourdoughs, it is more like one rise with gentle little interruptions.  

4. Shape dough into desired shapes (rounds, loaves, rolls, baguette). Place on parchment lined sheet pan or greased pan. This is when the oven should be preheated if you plan of baking right away.

5. Cover with plastic wrap and let proof for 4-5 hours (or over night in the refrigerator) until dough has doubled in size (the only time I got a second rise out of any of the attempts is when I let it proof in a VERY hot kitchen, as the AC had gone out and it was 105 degrees outside that day. It took about 6 hours, and it rose double, but looked weird, bubbly and with holes in it, though it kept its shape fine. Oddly enough, when it baked, it had no holes in it, and was very heavy and dense. )  I don't.  Here is where the dough sounds too weak to hold in the gasses from fermentation.  Also no mention of proofing temperature.  Instructions sound like a cold room with a weak starter.  With added yeast, the dough would be over-proofed.  With a strong starter, this would also not take so long.

6. With sharp knife or razor blade cut designs into the top of the bread dough. The cut enables the bread to rise evenly in the oven and for the crumb to open.

7. Preheat the oven to 475 F. Place pan with 1-2 inches of water on the bottom shelf of the oven. When water is visibly steaming, the oven is ready. Place the dough pan in the middle of the oven and reduce the heat to 425 f. Bake for 30-45 minutes.

I have used the bread flour with appropriate protein content (it was whole wheat, though) and a combination of rye and wheat in different ratios.With the WW and rye flours, I have added 1 tbsp gluten for each cup of flour I have used.  Every time, the bread has turned out very dense. no rise in the oven, and except for the last time, no rise with the proof. I have let it proof in the refrigerator over night for all but the last time, as it took so long for the first rise. The dough has doubled every time with the first rise. This is what bothers me.  The bulk rise with sourdough-only dough left alone until it "doubles."  Try reading about "Stretching and Folding" the sourdough while it is rising.  This gives you more chances to feel the dough and know what is going on during the fermenting process.  When the dough is stretched in intervals, the outcome is clearly seen in the resulting crust and crumb.