First, make the starter: Mix the cool water with the pinch of yeast. Then mix in the 125g flour. Cover and let stand at room temperature for 12-15 hours. It should have risen and appear bubbly.
Mix together the teaspoon of yeast with the lukewarm water (use less water in summer or in a warm, humid environment; use more in winter or a dry environment. If in doubt, use 250ml). Add the flour, all of the starter, and salt.
Mix and knead by hand till you have a soft dough that is mostly smooth. If using a stand mixer, knead for 5 minutes.
Grease a large bowl, and place the dough inside. Cover and let rise for 3 hours in total. After the first hour, deflate it and turn over. Do the same after the second hour.
Place the risen dough on a work surface that is lightly greased to prevent sticking. Divide the dough equally into three pieces.
Shape each piece into a rough baguette shape, then cover with greased clingfilm and let rest for 15 minutes.
To make a true baguette, take each piece of dough and fold in half lengthways. Seal the edges with the heel of your hand and repeat. With the seam side down, gently roll out the dough into a long, slender baguette.
Place the baguettes seam-side down on baking trays lined with parchment, or baguette tins. Cover with greased cling film, and let rise for 90 minutes.
Preheat your oven to 230 C / Gas 8.
Using a sharp knife, make three slashes in each baguette at an angle. For a nice crust, sprinkle them liberally with warm water.
Bake the baguettes for 25-30 minutes, until they're a nice golden brown. Remove from the oven and cool on a wire rack.