Slice cucumbers into rounds, and thinly slice bell pepper and onion. Combine in a large bowl. Sprinkle on salt, celery seed, and mustard seed and mix well to combine and coat vegetables with seasoning.
Stuff vegetable mixture into a half-gallon mason jar or multiple smaller jars. Loosely place lid and set aside to rest for 1 hour to draw out liquid.
After 1 hour, remove lid. Do not drain liquid.
Whisk together the cider vinegar and sugar until sugar is completely dissolved. Pour over the vegetables. Screw on jar lid.
Place in the refrigerator and let pickle for at least 2 days before eating, but preferably longer for the best flavor.