This is so easy!!
Cut the bottom of the stalk off.
I score my bottom with a knife, although I don't know if it is necessary.
In a small clay pot, place a "basket" coffee filter. This keeps the dirt in the pot.
Fill pot with dirt and place celery stalk in the pot.
Water and place in a sunny window. Keep the soil damp - I usually put the water in the drip tray and let the plant "drink" what it needs. I check the drip tray daily and add water as necessary
Mine looked like this in 7 days.