1/4 cup salt
Thinly slice the cucumbers and onions. Fill a large bowl with the slices, add salt and enough water to completely cover. Let soak for approximately 1 hour.
Thoroughly drain the salt water away from the cucumbers and onions.
2 cups apple cider vinegar
2 cups water
3 cups sugar
2 tsp. celery seed
Combine first 3 ingredients in a saucepan and heat until hot but not boiling. Pour hot vinegar mixture over drained onions and cucumbers. Add 2 teaspoons of celery seed to the mixture. Let soak in the refrigerator a minimum of 2-3 hours, but overnight is best.