Boil the cranberries for at least 10 minutes — this will allow the berries to release the fruit's pectin. For maximum firmness, include the full amount of sugar in a recipe; it helps the sauce set. (You can use less sugar, but it won't be as firm.) Let the sauce cool at room temperature before chilling it.
As a very last resort, if your cranberries still aren't jelling, try adding a teaspoon of pectin and boiling for five minutes. Cranberry sauce keeps well in the refrigerator, so if you make it the day before, it'll have time to come together well before your guests arrive.
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Source: Flickr User slgckgc