These are rough measurements because my idea of a medium potato might not be your idea of medium, so you might need a tad more or less dressing.
First tip, use fresh potatoes and quality ingredients.
6 medium potatoes, russet or yukon gold are good choices
1 small sweet onion, chopped approx. 1/2 cup
2 stalks celery, diced approx. 1/2 cup
1 cup Hellmann's mayonnaise
1/3 cup Gulden's mustard
1/2 teaspoon salt
Wash and boil the potatoes until they are just fork tender (tender when poked with a fork). Drain and cool until you can comfortably handle them.
While you are waiting for the potatoes to cool slightly, mix the dressing ingredients in a small bowl.
Second tip, mix while potatoes are still slightly warm so they absorb more of the flavors.
Peel and cut into 1 inch cubes. Add to the serving bowl and toss gently until coated.
I'm usually not brand conscious, however, this is one of those times when it has to be Hellmann's and Gulden's.