Put a small pot on medium heat and add the cranberries with the orange juice. Add the zest of the orange and the juice of the orange too. Bring to the boil, then gently simmer for about 10min until the cranberries have softened and started to burst.
Add the sugar and port and stir until the sugar has dissolved. Continue simmering the mixture for another 5 minutes until the sauce has thickened.
Please note the sauce will thicken further as it cools.