This is what I did a couple of years ago. I got an excellent starter out of it, and used it almost every day for about a year before getting too busy and putting it in the freezer. I think it's time to pull it out and get it going again.
Good luck with yours. There are a million ways to do it, but if you use a commercial yeast, you won't be culturing the sourdough yeast, you'll be growing more of the commercial yeast. I tried many different ways of doing it over the years, and this is the only one that was successful. Also, many recipes have you throw out a bunch of it as you're building it. If the volume gets too big, just use some of it in your regular pancake recipe or bread or something. I hate to waste food, and there's no reason to!
They do say that a bit of rye flour added into your first mixture helps, as does whole grain flour, but I was quite successful with plain old chlorinated tap water and all-purpose flour from the store. The starter got extremely active after a couple of weeks, and my bread could rise in just a couple of hours after shaping. I also used it in muffins, biscuits, cakes, cookies, anything breadlike!
Sourdough Starter
2 T. water (some people say non-chlorinated, I just used regular tap water, its fine.)
2 T. flour (use whatever flour youre going to be using most with the sourdough)
Mix together well in a clean glass or plastic container. Some people claim that using metal utensils with sourdough will kill it, but Ive always used metal spoons and forks with it and it is extremely active and healthy.
Cover loosely and place in a warm spot. On top of the refrigerator is a good place if the rest of your kitchen is cool. I kept mine near the stove, just not near enough to heat it and kill it. Do NOT store it on the stove, when you use the stove or oven it will overheat and kill it.
Every day, add 1 T. water and 1 T. flour. Mix in well, and pour into a new clean container. The container switching is very important to avoid mold contamination, which will grow on stuff splashed up the sides and left for a couple of days.
It may smell pretty nasty on some days, but just continue with the feeding and switching. After about 1 ½ weeks, it should begin bubbling and foaming after you feed it and leave it for a few hours, and it should have a fairly strong sour smell. At this point its ready to use. If it doesnt start foaming and growing up the container, dont despair, just continue feeding it every day and itll get there. It will usually have a liquid on the top after it sits for a little while, just stir that liquid back in.
Once its going well, you can put it in the fridge in a clean container and just leave it alone. Feed it every 3 weeks or so, using about half of it and adding back in ½ cup water and ½ cup flour, (thats what I do, though some people use 1 cup water and 1 cup flour each time. If you want to have it ready to use at a moments notice, you might want to do the 1 cup thing so youve got enough, but I usually just grow some more in it overnight if I know Im going to use it the next day.)
Good luck with it. I'll try to check back on this thread if you have any questions, or you can pm me. I love cooking and food experimentation!