The steps are few and simple: to obtain your sauce, cook the cranberries in hot butter for about 5 minutes, stirring constantly.
Slowly add the other ingredients, and cook over low heat for another 10 minutes.
The times could be slightly different if you use frozen ones: in any case, continue to cook until the blueberries are soft.
As soon as your mixture is ready, pour it immediately into sterilized jars, which you will then close immediately. Leave the jars upsidedown for 10 minutes, after which you can turn them over again and let them cool.