Why waste a perfectly good halter?!! And add to a landfiill!
Washing machine - with towels or rugs so as not to damage the washer, bucket and elbow grease or pressure washer.
Didn't say throw it out. I keep several, "Good" halters that I don't use daily. The others get hung, with a lead rope, various places around the farm. That way I always have a halter and lead rope handy. But the daily ones are not taken off the farm, I use the 'good' halters for stuff like that.