I learned this from my grandma, who never went by a recipe (a little of this and a little of that), so I don't have exact amounts for the ingredients. But, this is an easy one to do by taste.

Cooked macaroni
Velveeta cheese, cubed
Sweet onion, chopped
Celery, chopped
Mayonnaise or Miracle Whip
Dijon mustard

The only trick is too add the velveeta to the macaroni while the macaroni is still warm, so the cheese melts. Then add the other ingredients and salt and pepper to taste.