Below I give amounts for all ingredients, the thing about potato salad is that depending on what potatoes you use, you might need more mayonnaise and salt so just add the below amounts and add more if needed.  You don't want this swimming in mayo but it does hold it together

Ingredients
500g potatoes, cut into small chunks - I like Vivaldi
3 tsps salt
1 white onion, finely chopped
6 - 10 tbsps of mayonnaise
¼ tsp white pepper

Method
Put the potatoes and 2 tsps of the salt into a large pot and fill with water.  Put the water on a medium high heat and bring to the boil.  Once the water is boiling, turn down to a simmer and cook until the potatoes are tender, this usually takes around 15-25 minutes so test at 15.  The potatoes need to be tender but not overcooked or they will break up.

As soon as you are happy with your potatoes, drain in a colander and immediately put your onions in to the warm pan and leave for a couple of minutes as the residual heat will soften them. 

Put 6 tbsps of the mayonnaise in along with the remaining salt and the white pepper and mix with the onions.  Add the potatoes and carefully turn until they are covered in the oniony mayo. 

At this point if the potatoes look a little dry add some more mayonnaise until you are happy.  Eat warm (which is how I love it the most) or leave to go cold.