When I make it, I make it according to the dressing recipe but I cut the mayo and buttermilk by half a cup each. So, it comes out to:
1 packet Uncle Dan's Classic Ranch mix
1 1/2 C Mayonnaise
1 1/2 C lowfat Buttermilk
Combine ingredients in a large bowl and mix well. Chill 1-2 hours prior to serving.
You can also make it into a dip by using mayo and sour cream, but I find that it's just as good making it as if it's dressing and using it for a dip anyway.