Roast the tomatillos and garlic (unpeeled) on a skillet or comal until they are well cooked. They should be soft and almost burnt.
Put the peeled tomatillos and garlic in a blender, and add one or two chipotle chilies.
Blend everything and taste. Adjust seasoning if necessary.
Notes
To store canned chipotle chilies in adobo more easily, I like to make a paste: how to make a chipotle paste.
Store the sauce in a well-sealed glass jar in the refrigerator. It will last for up to three weeks.