It's apparently quite simple: just stand the base in a small dish of water for a week or so until new leaves appear in the center. The leaves will be yellow at first, and once they really emerge and turn green, you can plant the celery in the ground or in a pot.
Growing celery from seed can be a challenge. Celery has a long growing season and takes a while to develop when sowing from seed. Here are some tips on how to start celery plants from seed and using self-watering containers to maintain the moisture levels needed for the plants to thrive.