Cut off the tops of the jalapeños. Remove seeds if you want it to be mild! Combine all ingredients in a blender. Blend to desired consistency. That's all! As for the ingredients, I've always just sort of eyeballed the amounts. You can likely do the same, but I added more specific amounts for those of you who are a little on the safer side.

To make a chunky salsa: Blend tomatoes (without the juice), and chop up the onion and jalapeños fairly small. The onions can be a little bigger than the jalapeños. Add the onion and peppers to the blended tomatoes and add the cilantro (this time, you won't want the stems, so be sure to cut them off).