Dry-fry the cumin and coriander seeds in a small, heavy-based frying pan over a medium heat for 2–3 minutes until they emit a nutty aroma, then grind them to a powder in a mortar with a pestle.
Drain the chillies, chop off the stalks, and squeeze out most of the seeds. Discard the stalks and seeds and coarsely chop the chillies. Using a mortar and pestle, pound the chillies with the garlic and salt to form a thick, smooth paste.
Add the ground spices to the paste and pound again, then beat in most of the oil.
Spoon the paste into a sterilized jar and pour over the remaining oil. Seal the jar and store in a cool place or in the refrigerator. Use within 2 months.