In medium-size bowl or a large measuring cup, mix together all ingredients. Mix until fully-combined and smooth. If you want the dressing to be thicker, add more mayonnaise; if you want the dressing to be thinner, add more buttermilk.
Store in a mason jar in the refrigerator until ready to serve.
Notes
If stored in an air-tight container (such as a mason jar), homemade ranch dressing will keep for 1-2 weeks in the refrigerator.
Once stirred, if you want the dressing to be thicker, add more mayonnaise; if you want the dressing to be thinner, add more buttermilk.