Crumble 12g of fresh yeast into a bowl containing about 30g of White Bread Flour. Use a fork or your fingers to mix it up into a dry crumbly mixture then put into a small plastic bag and place in the freezer. When you want to use the yeast from the freezer, tip the mixture into a bowl, add 100ml of tepid water and a very small pinch of sugar and set aside to start working. As it has been frozen it may take a little while to become active.
Once you have created an active starter it can be a good idea to freeze some for future baking sessions. To do this, measure 100g active, lightly bubbly starter into a freezer container, add one tablespoon of wholegrain flour and one of water, stir, cover, label and freeze for up to 6 months. To use the frozen starter, take it out of the freezer and thaw at room temperature, about 3-4 hours. Add 100g flour and 100ml water, mix well, cover and leave until active and bubbly, about 6-8 hours.