I found this on epicurious.com when trying to find a recipe to use up one lone egg yolk. I hope you find it helpful when you find yourself in the same predicament or looking for a good homemade mayo. Can be made up to 2 days ahead.
Combine egg yolk, lemon juice, vinegar, mustard, and 1/2 teaspoon salt in medium bowl. Whisk until blended and bright yellow, about 30 seconds.
Using 1/4 teaspoon measure and whisking constantly, add 1/4 cup oil to yolk mixture, a few drops at a time, about 4 minutes. Gradually add remaining 1/2 cup oil in very slow thin stream, whisking constantly, until mayonnaise is thick, about 8 minutes (mayonnaise will be lighter in color). Cover and chill. Keep chilled.