Diced celery is useful in many recipes - it has a distinct flavor, and is popular in stews, soups, salads, and even garnishes. Use this technique to practice a fine dice, and you'll have a foundational knife skill to utilize within recipes.
Separate the stalks from the bunch. Wash the celery, then trim the ends off, discarding about 1" of the root end.
Cut the stalk into thirds, about 3" pieces long. With each piece, vertically cut 3-4 long thin strips. Rotate the strips 90 degrees, then finish the dice, sliding your knife up and down to cut desired size dice of celery. When you're finished cutting, use a bench scrapper to pick up the diced pieces and place them in your container.