Wash and dry celery leaves and other celery bits and pieces. (To reduce drying time, consider chopping stalks into smaller pieces.)
Fully dry leaves and stalk pieces in a dehydrator or at the lowest heat and convection setting of your oven. (Check occasionally and remove dry, brittle pieces, leaving the rest of the celery pieces to continue drying until brittle.)
Once they are fully dry, grind up the celery leaves and stalk pieces until you obtain a fine powder. Celery leaves can be easily pulverized with a mortar and pestle. If using stalk bits, though, I find it much easier to use a spice grinder of coffee grinder.
Store the homemade celery powder in a well-sealed container to keep moisture out, preferably out of direct sunlight.