If making mayo from scratch, do so first in a large mason jar. You can then add all remaining ingredients into the jar. Otherwise, just mix all ingredients together in a medium bowl.
Adjust the amount of milk you use according to the desired consistency. For a ranch dip, you may only need to use 1/8 of a cup. For a thinner dressing, you may use the entire 1/3 of a cup. Start small and add as you stir if needed.
Store in a jar in the fridge. You can use right away, but it's best if you wait at least 4 hours for the flavors to meld together.