Remove the stalks from the leaves, place in single layers on mesh lined dehydrator trays. Dry at 135° till dry. Time will depend on humidity in your area. Mine took a few hours, we have very dry air right now though.
At the same time I finely chopped the stalks from the leaves and then finely sliced the stalks of celery. I placed them on trays as well and dried them alongside. They took overnight. Once the leaves are dry, remove them and let the stalks keep drying.
Once all is dried, let cool down then pack into glass jars for storage. The big jar is leaves, the little jar is the celery. Celery is nearly all water, it shrinks down considerably.
Crush the leaves as needed for powder, the celery can be broken into small pieces if desired for faster rehydration.