In a pot, add all ingredients and bring to a boil.

Turn heat down to medium-low and simmer for 15 minutes. 

Use a spoon so smoosh cranberries into the side of the pan. 

Remove from the heat. 

Use a ladle to spoon some of the mixture into a mesh strainer. Use a spoon to press the mixture down, which forces the juice out of the mixture. Do this until the mixture is firm and has little moisture.

Allow the syrup to cool.

Add 3 ounces rum to a glass. Pour ¼ cup syrup into the glass. Top off with 7UP®.