Step by step

Add plenty of oil to the mortar and continue to pound until you obtain a creamy and fairly fluid sauce

Tips
The pesto can certainly be made in advance: but because of its tendency to oxidize, you have to make sure you cover it with a little extra virgin olive oil.
Trivia
The pesto is made using a mortar and pestle - hence the name - generally in this order: first pound the garlic together with the pine nuts, then add the basil leaves.