Lingonberries are to Swedes is what ketchup is to Americans! Almost! Lingonberries are one of the most common condiments in Sweden, used to garnish fish and meat dishes as well as pastries and desserts.
I recommend that you use frozen lingonberries. It simply doesn't turn out as good with fresh ones, at least not when making preserves. So, one 400g bag of lingonberries: pour about 1 to 2 dl sugar over the berries. Leave it at room temperature for about 5 or six hours, stirring every hour or so until the sugar has melted. Done. This should keep in your refridgerator for up to 2 months.