basic mayo

1/2 teaspoon vinegar
salt to taste
light olive oil or other flavourless vegetable oil
egg yolk
a dollop of mustard

put mustard, pinch of salt and egg yolk into a bowl and start beating

SLOWLY drizzle the oil into the bowl whilst continuing to beat
at the beginning almost a few drops of oil at one time 

mixture will slowly turn from egg yolk yellow to a paler yellow

add quarter teaspoon vinegar

taste and decide if another quarter teaspoon of vinegar would help