I just used, "pure water," as a general term. Personally, I have natural, spring water at my house and so I just use that. It does not have any additives in it and comes straight from the ground. Well water is probably fine too, provided it doesn't smell sulfury, or have any unwanted elements in it. I would not use regular tap water because of the added chemicals. If that's what one has, then I would buy some purified, bottled water for the project.
I've been making and keeping starters for ever and I haven't really measured anything in ages, so please note that these are just estimates:
1 1/2 Cups of white flour
1 1/2 cups of water
1/8 teaspoon white sugar
Mix all ingredients together thoroughly and put in a 1 qt. mason jar. The starter should have the consistency of thick pancake batter. Adjust the amount of water and flour if it does not. Cover with a cloth and set in a warm area of the kitchen, (around 70-78 degrees F.) Let sit covered, for a few days, stirring once, or twice a day. After 5, or 6 days, you should start seeing bubbles forming in the starter. This means that the yeast has moved in and started growing and reproducing. If not, stir in another 1/8 teaspoon of sugar and wait to see if there are any bubbles. If there's still no foam action, or you see blueish green mold growing on it, then I would throw it out and start over. If there are bubbles forming then put a lid on your jar and move the starter to the refrigerator until you are ready to use it. The more it is used the better it will get.
*Note, some recipes will call for some regular, baking yeast to be added to the flour/water mixture. That's OK too, but then, you will not have the local, wild yeasts dominating your starter. (After all, that is what the San Francisco sourdough is so famous for. It is their local yeasts, among a few other things, that give the bread its unique taste and texture.)
Good luck!
~ C