Try this recipe for homemade mayonnaise.
Making mayonnaise at home requires nothing more than a blender and ten seconds of high-speed initial mixing to properly distribute the egg and dry ingredients before the fat is introduced.
Excessive pouring speed overwhelms the egg proteins and breaks the emulsion, resulting in an oily liquid rather than a thick spread. Temperature control also matters, as room-temperature ingredients bond more reliably than cold components straight from the refrigerator.