1/2 cup of grated Horseradish root
Apple Cider OR Wine Vinegar. This recipe does not lend itself well to distilled white vinegar.
Optional Add Ins
Garlic - Don't get carried away or it could commandeer the batch, just a clove diced or crushed will suffice.
Chives, pepper corns, mustard seed, use your imagination but just as with the garlic don't let the add ins dominate. The idea is to meld the flavor of the horseradish into the vinegar.
Peel then grate the Horseradish. A cheese grater works dandy for this task. You could also dice and slice the Horseradish root and process in a food processor / blender.