Slice the grapes and combine with the flour and the water.
Tip the dough into an airtight container. Cover and leave to rise for three days at room temperature.
After three days, discard half of the mixture and 'feed' the remainder with another 100g/3½oz of flour and 100ml/3fl oz of water. Stir well until the mixture is thoroughly combined.
Leave for at least 24 hours and the starter should be ready to use. It should be like a bubbly jelly in texture. If it is not yet bubbling and active, feed it once more and leave it for another couple of days.
If you are using the starter regularly, say every week, you need to discard half and re-feed it every couple of days. Also, feed the starter after you make each loaf of bread. The starter should be stored at room temperature (except on very hot days) and should be a like a thick batter.
If you are only making bread once a month you can keep it in the fridge but return it to room temperature before using. Keeping it in the fridge will slow down the activity but not stop it. If the starter seems to be inactive, give it another feed. Remember it is a live mixture of yeast and bacteria and it needs to be fed.