wash and de-stem all the chiles

cover with hot or boiling water and place a plate on top to keep submerged

allow to rehydrate for 20 minutes

Remove with slotted spoon, retaining soaking water

Put chiles, salt, oregano, and garlic in blender, and blend until fine

Add tomato sauce (save can for transferring water), and blend again

Add soaking water until the thickness you want is achieved (about 2-3 cans worth)

Blend for a minute or two until smooth