Make the starter by mixing the yeast with the water (no need to do this if you're using instant yeast), then mixing in the flour to make a soft dough. Cover and let rest at room temperature for about 14 hours; overnight works well. The starter should have risen and become bubbly. If it hasn't, your yeast may not be working. Dissolve 1/4 teaspoon of yeast in 1 tablespoon lukewarm water with a pinch of sugar, and wait 15 minutes. If nothing happens, replace your yeast, and begin the starter process again.
If you're using active dry yeast, mix it with the water, then combine with the starter, flour, and salt. If you're using instant yeast, there's no need to combine it with the water first. Mix and knead everything together—by hand, mixer or bread machine set on the dough cycle—till you've made a soft, somewhat smooth dough; it should be cohesive, but the surface may still be a bit rough. Knead for about 5 minutes on speed 2 of a stand mixer
Place the dough in a lightly greased medium-size bowl, cover the bowl, and let the dough rise for 3 hours, gently deflating it and turning it over after 1 hour, and then again after 2 hours.
Turn the dough out onto a lightly greased work surface. Divide it into three equal pieces.
Shape each piece into a rough, slightly flattened oval, cover with greased plastic wrap, and let them rest for 15 minutes.
Working with one piece of dough at a time, fold the dough in half lengthwise, and seal the edges with the heel of your hand. Flatten it slightly, and fold and seal again.
With the seam-side down, cup your fingers and gently roll the dough into a 15" log. Place the logs seam-side down into the wells of a baguette pan; or onto a lightly greased or parchment-lined sheet pan or pans.*
Cover them with a cover or lightly greased plastic wrap, and allow the loaves to rise till they've become very puffy, about 1 1/2 hours.
Towards the end of the rising time, preheat your oven to 450°F; if you're using a baking stone, place it on the lowest rack.
Using a very sharp knife held at about a 45° angle, make three 8" vertical slashes in each baguette. Spritz the baguettes heavily with warm water; this will help them develop a crackly-crisp crust.
Bake the baguettes until they're a very deep golden brown, 25 to 30 minutes. Remove them from the oven and cool on a rack. Or, for the very crispiest baguettes, turn off the oven, crack it open about 2", and allow the baguettes to cool in the oven.