Scale the ingredients according to the amount of water - the ingredients list is based on 2.5 litres of brine.
Put the same amount of water (or a little less if you are going to cool the brine with ice) into a steel or enamelled pot, and add the rest of the brining ingredients, and heat slowly on the stove while stirring until the ingredients, until the sugar and salt have dissolved.
COOL THE BRINE! The brine should not be warm when adding to the turkey. I add a tray-full of ice to the measuring jug, then add another tray to the pan - stirring both until the brine is cooled. Or you could just cool it in the fridge.