I haven't made a cheddar with cranberries myself, but I know it's a popular combination. When you're adding fruit to cheese, you should use dried cranberries instead of fresh ones. This is because fresh fruit has a lot of moisture and sugar, which can mess with the aging process and cause the cheese to spoil. Add the dried cranberries after the curds have been cut but before you press the cheese. This will make sure they are spread out evenly. Just be careful not to overdo it, as too many can affect how the curds knit together.