[header] How to make vinaigrette [title] Add a small amount of mustard to a small bowl. [step] You should add a dollop of mustard that is about the size of your fingertip. Mustard acts as an emulsifying agent--when two liquids don't mix, like water and oil, mustard breaks them down and helps them to mix. 
[substeps] You can also add a fingertip-sized dollop of mayonnaise to the bowl. Mayo is also an emulsifier and adds a nice creamy texture to the vinaigrette.