You have saved some of your yeast culture and put it in the fridge, keeping that as a starter for the next loaf and can now prepare the dough !

Apparently salt improves the texture of the loaf, so add a teaspoon thereof to the bowl. (I don't do this myself, as one of the reasons for my doing sourdough is to avoid salt where possible, and there is still a good deal of flavour.) Then add flour until the mixture is too stiff to beat with a spoon.

Spread a little flour on a work surface, and put the dough onto that. Knead the dough, stretching it and rolling it back on itself again and again, adding flour as required until it is not horrendously sticky any more. Bear in mind that a pure wholewheat loaf will require lots of kneading - I suggest 10 minutes as being an ideal time - a dough made using white flour will need less time.

Letting the dough rise: here I break somewhat with convention: The usual practice is to let the loaf rise until doubled in size and then knock it down, reshape it and then put it into a greased bread pan for the final rising.(I use unsalted butter to grease the pan, but salted butter should be fine)

I prefer to let it rise once, directly from the first kneading, and then bake, thereby saving time. The main reason is that I often start the kneading process after coming home from work and having supper, but I still want to get to get a good nights' sleep, and the bread is perfectly tasty anyway. (Another factor you might want to consider is that not all sourdough yeasts can handle that second rising.)

Allowing the dough to rise twice might create a better texture ? - try the 2 techniques and see which works for you. The loaf will generally take between 4 to 8 hours to rise, depending on the warmth of the environment and the type of yeast (remember wild yeasts from around the world each have their own characteristics).