If you are using them in beer making, then they are used at the rate of 1/2 a crushed campden tablet per 25 litres of water to remove chlorine/chloramines before any other process.
If you are using them in winemaking they are added to fruits/vegetables as a yeast inhibitor 24 hours before fermentation on the pulp at the rate of one tablet per 5 litres, they are also used at the same rate after each racking :thumb: