These instructions will give a loaf to serve around lunchtime. If you'd like to serve it for dinner, make the poolish in the evening, mix the main dough in the morning and pour into the baking tin. Place it in the fridge for at least 10 hours, and then cook.
2 cups bread flour
500mls water
Pinch of yeast
Main dough
2 cups bread flour
150mls warm water
2 teaspoons salt
1 teaspoon yeast
The night before mix the poolish ingredients together in a large plastic container with a lid. You may need to use your hand (washed, of course) to mix the poolish well. When finished scrape the dough off your hand back into the container.
Place the lid on, leaving a small opening at one corner. Leave out of the fridge overnight.
The next morning add the main dough ingredients to the poolish. Again, you’ll probably need to use your hand to do this. It can take up to 5 mins to get it mixed in properly. The dough will be lumpy at this point, and very slack, but all the flour should be well moistened. It will be too wet to knead.
Let the dough rest for 10 mins. Then wet one hand (this stops the dough sticking to your hand) and slide it half way under the dough. Lift the dough up until you feel resistance, then fold this part of the dough back over itself. Turn the container 90 degrees, wet your hand again and repeat. Do this a total of 4 times, almost forming a square. (Though the dough will not look square. It will be round.)
Let the dough rest another 10 minutes and repeat. And then rest another 10 mins and repeat. So you do the stretch and fold proceedure a total of 3 times. This aligns the gluten strands and you will see the dough become smoother the more times you do it. This all sounds way more complicated to describe than to do. It’s actually quite simple. You just make a square with the dough 3 times.
Line a shallow baking tray with silicone paper.
Allow the dough to rest a few more minutes, then plop it onto the baking tray. Wet your hands for a final time and push and smooth the dough to reach the ends of the baking tray. If the dough seems reluctant to stretch, let it rest a little longer. Press your fingers deep into the dough, touching right to the bottom of the pan. Do this about 10 times to make 40 dimples. This is what gives foccacia it’s charachteristic lumpy appearance.
Set aside to rise for anything from an hour to five hours. If you want to leave it longer than that you could refrigerate it for part of the time. No need to cover it.
An hour before you want to bake it, preheat the oven to 250 degrees.  
Drizzle 3-6 tablespoons of olive oil over the top of the dough, being careful not to actually touch and therefore deflate the dough. I like to sprinkle it with rosemary too, but this isn’t necessary
Bake for 25-35 mins, until golden brown on the top. Check it after 20 mins - you may need to rotate the pan if it’s cooking unevenly.