The best way to steam a Christmas pudding is to wrap it in a pudding cloth and then tie it with string. Make sure the pudding cloth is big enough to cover the entire pudding, leaving a bit of space around the edges. Place the pudding cloth in a large pot and add enough water to cover the pudding by at least 1 inch. Bring the water to a boil and then reduce the heat to a simmer. Steam the pudding for 2-3 hours, checking the water level occasionally and adding more if needed.