I have a couple recipes for Ranch Dressing. This first one is from my sister, it's a very good version.
Ingredients
1 cup mayo
1/4 cup sour cream
1/2 cup buttermilk
1/2 tsp dried chives
1/2 tsp dry parsley
1/4 tsp garlic powder
1/4 tsp onion powder
1/2 tsp dill
Instructions
Basically - mix it all together. Let rest for a while so the flavors can meld.
This next one is from Serious Eats, I haven't tried it yet. This is a PDF.