I like to use 2 yolks for a couple different reasons; one, I like the richness of it and two, I find that it emulsifies easier. I also like to blend mine by hand, you can use a food processor but that's just the way I was taught. Now I think classically you want to use the dijon, but for me it just depends on what I am using it for and what I might want to flavor it with later.
Create you emulsification by whisking together the yolks with the dijon(if used) and salt
If you want the garlic to have a stronger flavor I would start with it now. In terms of the garlic make a paste with the back of knife or mince it super fine.
Slowly add in your oil. As the emulsification becomes stronger, you can start to add the oil faster, making aioli is not nearly as sensitive as making hollandaise
As your mixture begins to thicken you will want to thin it out. At this point add in your lemon juice and taste. The ratio of oil and egg is a balanced flavor between the two. They say one cup oil to one yolk but I find I like to taste that. I am also one that doesn't follow recipes so if you are, 1:1 is your best bet. (If you following this recipe use 2 yolks and 1cup oil.)
Chances are that you will need more oil and it will thicken up again, at this point add in a splash of water. When you get your desired thickness add in the garlic for that less potent flavor
Remember if you are going to flavor you aioli with something that is liquid based to account for that in your thickens. And if you want to just make plain ole mayonnaise omit the garlic all together.