Start by chilling your oil in the fridge, as this aids the emulsification process. Meanwhile, peel your garlic cloves and remove the germ, if present.
Put the garlic cloves and salt into a high-speed blender or food processor, blending until smooth.
Now comes the important part - slowly add the oil while the blender is on a low-medium setting. Add lemon juice after every ½ cup of oil. Make sure to scrape down the sides occasionally.
Continue this process until all ingredients are combined and you're left with a creamy, mayo-like sauce.