Crush spices lightly to break them up into small fragments, to minimise stress on your spice mill or food processor. Transfer them to the mill or processor, then blend on medium speed for 15–20 seconds, until reduced to a fine powder.
If spices are not ground fine enough by this time, wait for a few minutes to let them cool slightly—they warm up during blending because of friction—then pulse-blend until ground fine.
Sift powder through a fine-mesh metal sieve, then transfer to an airtight glass jar. Cover and let stand in a cool place for at least 1 week before using to let the aromas blend.