Remove the stems from the Thai chilis, and roughly chop. Mince the garlic cloves, and chop the cilantro.
Add the Thai chilis, garlic, cilantro, fish sauce, sugar, and lime juice to a small food processor. Pulse the food processor about 10 times to break down the mixture.
If using a mortar and pestle, crush the chilis, garlic, and sugar until a rough paste forms, about 3 to 4 minutes. Then, stir in the fish sauce, lime juice, and cilantro. Serve, and store in the fridge for up to 4 days.