Place the garlic in a mortar along with the salt.
Using a pestle, smash the garlic cloves to a smooth paste. (The salt stops the garlic from slipping at the bottom of the mortar as you pound it down)
Add the lemon juice to the garlic.
Drop by drop, pour the olive oil into the mortar slowly as you continue to crush the paste with your pestle.
Keep turning your pestle in a slow, continuous circular motion in the mortar.
The drip needs to be slow and steady.
Make sure the paste soaks up the olive oil as you go.
Keep adding the oil, drop by drop, until you have the consistency of a very thick mayonnaise.
If your allioli gets too dense, add water to thin it out.
This takes time, around 20 minutes of slow motion around the mortar, to create a dense, rich sauce.