With a damp paper towel, wipe off any dust on the peppers.
Remove the stems, seeds, and veins of the guajillo peppers
Add 6 cups water and add the onion and garlic.
Put the lid on, and bring to a boil.
Add the guajillo peppers.
Use a fork or tongs to submerge the chiles completely.
Turn heat off, leaving the ingredients inside.
Wait for 5 minutes, or until the chiles are pliable.
Discard the water.
To a blender, add the rehydrated peppers, garlic, onion, cumin, and salt.
Add 2 cups water to the blender.
Blend until smooth.
Carefully strain the sauce into a large bowl, using the back of a spoon.
Taste for salt.
Store in the fridge until ready to use.