The first thing you need is a ladleful of your sourdough starter. Pour this into the biggest bowl you own, and mix in a glass of water. Add a good teaspoon of salt - if you're using Maldon that's two teaspoons (before you do any crumbling), then add around four ladlefuls of strong white flour. You could well need a bit more, or maybe a bit less, but stir it with a sturdy knife until it's all incorporated. If it's sticky add some more flour, and you want it to be soft so you can pull it into short ribbons, but not so sticky that you can't quickly pick it up (you won't be able to pick it up and let it sit in your hands as it'll be too sticky).
Knead in a tablespoon of sugar if it's cold, or if you're feeling a bit nervous about the strength of your sourdough starter.
If it's not the summer put your oven on the lowest it'll go, so it's only just warm, and leave the dough there in a bowl for an hour or two, covered with clingfilm that doesn't touch the surface of the dough.
Take the dough out and give it a good knead for around 5-10 minutes, the last few kneads with plenty of oil on your fingers to really stretch it out, then put it into a clean but oiled bowl. Back in the oven, if it's cold, or just left on the side with clingfilm over it, for several hours. It'll be quite happy like this even if left overnight (in which case don't put it in the oven).
When it's looking puffed up, maybe half as big again as the size you started with, hike up the oven temperature to maximum, fan on if you have one, and put a heavy tray in the oven to heat. Once your oven is as the hottest temperature, put some polenta or flour onto the tray, carefully tip the dough onto it, gently slide back into the oven and gently close the door. A shallow dish on the floor of the oven, with boiling water poured into it before you start baking, will make things much more crispy. Depending on how big your loaf is, 25 minutes later check for a hollow-sounding bottom. Don't be afraid to put it back in for another 5-10 minutes, especially if it's big. You want it brown and crisp.