Line the base of a large stockpot with a clean tea towel, then place the jars in, standing upright, and cover with cold water. Bring to the boil, then simmer for 10 minutes. Turn off the heat, add the lids to the pan, cover and leave to stand for 10 minutes. Using tongs, remove the jars and lids from the pan and drain on a clean tea towel. Fill while the jars are still warm, then seal immediately.