First, put 1/2 teaspoon of Caldwell's starter culture in a small glass bowl and pour 1/4 cup filtered water over it. (In place of this you could use 2 tablespoons whey or leftover brine from another ferment. Be sure to choose one who's flavor will complement the horseradish if going this route. Also, if you don't want to use any starter, you can just add more salt (3 T). It may take longer to ferment, however.)
Peel the horseradish.
Next, grate the horseradish root. I used the grater function on my food processor, but you can do it the old fashioned way as well.
Pack your sterilized mason jar with your shredded horseradish. Be sure to leave an inch of headspace to allow for any expansion during fermentation. Then pour the culture/water over it. Add your fermentation weight (or sterilized rock) and then top the jar off with filtered water to cover it.
Finally, after putting an airlock lid (or regular mason jar lid) on your ferment, set the it on your counter out of direct sunlight for at least 3 days. I like mine when it has fermented for about 1 week. Keep tasting it out after the 3 day mark to see when it gets to your liking.
Finally, use or refrigerate. Your ferment will keep for months, but it will continue to ferment (albeit slowly) while under refrigeration. It may become more spicy or less spicy. I have had both happen!