I got the recipe from here, but this is my halved-down version.
1 cup light olive oil
1 tablespoons apple cider vinegar (or fresh lemon juice)
1/2 teaspoon fine celtic sea salt
I also add
1/2 teaspoon dry mustard
1/8 teaspoon cayenne pepper
Combine the egg, vinegar, salt, mustard, and cayenne in a blender and pulse a few times until frothy.
With the blender running, add the olive oil a little at a time – almost drop by drop at first – until an emulsion starts to form.
Continue adding the rest of the oil in a slow, steady stream.
Add more salt or other ingredients to taste.
Store, tightly covered, in the refrigerator for up to 7 days.