My preferred method is to take a pint jar, fill it about half way with buttermilk, add the seasoning mix, and shake to blend well. Then I add the mayonnaise and shake or stir until it is completely mixed.
If you use sour cream instead of buttermilk, you have chip dip.
Read about a few good reasons to make your own ranch in this post from August of 2015 How to Make Ranch Dressing