How to Make Fresh Horseradish
Put into a food processor. Add a couple tablespoons of water. Process until well ground. At this point be careful. A ground up fresh horseradish is many times as potent as freshly chopped onions and can really hurt your eyes if you get too close. Keep at arms length away, and work in a well ventilated room. Strain out some of the water if the mixture is too liquidy. Add a tablespoon of white vinegar and a pinch of salt to the mixture. Pulse to combine.
Note that the vinegar will stabilize the level of hotness of the ground horseradish, so do not wait too long to add it to the mixture.