Place a comal or cast iron pan over medium heat. Roast the tomatillos until cooked through, charred, and blackened on all side. Roast the garlic until cooked through with a few brown spots on the peel. Remove from heat when ready.
Molcajete method
With the pestle, mash peeled garlic, cumin and 1/2 tsp of kosher salt into a paste. Mix in guajillo and water until combined. Then, add one tomatillo at a time, mashing until smooth and well combined. Adjust salt to taste.
Blender method
Blend all ingredients until smooth, then stir in salt to taste.