You will need:
3/4 cup mayonnaise
3/4 cup milk
1 cup chopped celery
1 small red onion, diced
2 cups cooked chicken, diced (optional)
Combine dressing mix with mayonnaise and milk; whisk to blend. Chill at least 1 hour.
In a large pot, bring 4 quarts of water to a boil. Add pasta to boiling water and cook for 6-8 minutes. Do not overcook – pasta should be quite firm for salads. Drain pasta and rinse under cold water. Refrigerate pasta for 1 hour. Toss chilled pasta, dressing and remaining ingredients together; serve immediately.