On the first day, mix together the flour and water until it forms a thick paste, cover but loosely and leave in a warm place for 24 hours
The next day, stir the mixture then add the additional flour and water. Mix them in, then cover and set aside overnight again.
Repeat each day - by the third or fourth day it should be showing a good level of bubbles. It should be fine to use on the 4th day, but you can also feed again and use on the 5th day.
After you have taken some out to use for the loaf, you can either feed again to keep it alive and then keep taking some out as you need it to use or as it gets too big. Alternatively, you can cover and let it 'sleep' in the fridge. Then when you need it, take some out, feed it in the same way as above and it should come back to life after a day or two.
Notes
Note - recommend using mineral/bottled water or boiled and cooled water to mimimize chlorine in the water. Timing is active time, not the various wait times in between.