Place your cranberries in a colander and rinse well. Pick over them if you need to. Also, line a bowl or gelatin mold with plastic wrap. Set aside.
Put the cranberries in a saucepan and cover them with water. Bring the water to a boil over high heat and cook until the cranberries soften and burst.
While the cranberries boil, add the packets of gelatin to 1/2 cup of water. Gently stir until the gelatin dissolves.
Transfer the boiled cranberries to a food processor or food mill along with the sugar and dissolved gelatin. Process until smooth. Add the horseradish, give it a quick pulse and then taste. Not spicy enough? Add a little bit of horseradish at a time until it's perfect.
Transfer the jellied sauce to the plastic-lined bowl, cover with plastic wrap and chill at least overnight but no longer than 2 to 3 days.
When your cranberry sauce is firm and has some jiggle, you can turn it out onto a serving dish -and that's it. Enjoy!