This recipe is from Hugo Ortega’s Street Food of Mexico cookbook. Find several more recipes from the book here.

8 to 10 guajillos (long, reddish dried
   chiles), stemmed, seeded,
   reconstituted with enough water to 
   cover (reserve liquid)
2 dried chiles de árbol, stemmed,
   reconstituted 
6 garlic cloves, peeled, roasted 
1 tablespoon finely chopped white onion 
1 ¼ teaspoons kosher salt

Place guajillos and chiles de árbol in a blender along with ½ cup of reserved liquid, garlic, onion, and salt and purée into a smooth, thick but pourable paste. If necessary, add more reserved liquid, 1 tablespoon at a time.