Choose a substitute based on what you have: celery seed for closest flavor, dried celery flakes for similar body, or celery salt if you can control added salt.
Measure by the listed ratio (or start slightly under if your substitute contains salt).
Stir into hot liquids early for flavor release; for blends, mix with other seasonings before adding.
Taste at the midpoint of cooking and adjust: add more for flavor, or balance with a pinch of sugar/acid if it turns sharp or too bitter.
If using coarse flakes or seeds, crush/pulse for a finer texture so the substitute dissolves more evenly.