Put celery seed in grinder. See notes above about grinder options.
Grind celery seed to desired consistency.
Mix with salt.
Store in a closed jar in a cool, dark place.
Notes
Kosher Salt: You can also use kosher (coarse) salt. Just use twice as much salt and grind it together with your celery seed.
Shelf Life: Celery Seed has a super-long shelf life - so you can go ahead and buy it in bulk. Whole spices typically have very long shelf lives, while ground ones lose their potency and freshness more quickly.
AIP: For an AIP version (or simply another option) of this recipe, use celery leaves instead of celery seed. The proportions are up to you, but one option is about 1 tablespoon of sea salt to 6 tablespoons of celery powder.