Then, you slowly – keyword, slowly – incorporate the oil. I found it was easy to control how much oil I was adding by mixing the ingredients by hand.
You really need to add the oil in drop by drop, or else your mayonnaise won't thicken. Then, when you're halfway through your oil, gently pour the rest of it into your bowl in a slow, steady stream while mixing.
Before long, I had something that resembled mayonnaise.