Place all ingredients EXCEPT oil in the immersion blender beaker. Allow them to come to room temperature. This is an important step. Cold ingredients will not work. Not at all. Not even a little bit. Don't risk it. Stop. Put the blender down. Go watch TV, take a walk, fold the laundry. Do something but do NOT try to blend your mayo.
When everything is nice and room temperaturey - blend for 10 seconds or so until thoroughly mixed.
Slowly, slowly, slowly add in the oil while blending. You may need the whole cup - you may not. If the mayo gets to the consistency you like - STOP adding oil. If you add too much there is no going back!