Preheat oven to 400 degrees F.
or
Preheat oven to 205 degrees C.
Place chilis on a baking sheet. Toast lightly in oven, 3 or 4 minutes, until they give off a mild aroma. Do not burn, otherwise, chilis will be very bitter.
Remove chilis from oven and set aside until cool enough to handle.
Remove and discard stems, seeds, and any pink pithy materials.
Rinse chilis in cool water, drain briefly, then cover with hot water and soak for an hour.
Process chilis in blender with enough of the soaking water to obtain a nice, blended consistency.
Add remaining water, tomato sauce, garlic, oil, salt, oregano, and cumin. Simmer, uncovered, for 10 minutes, stirring occasionally.
Sauce can be refrigerated for up to 1 week or frozen for later use.