First things first: use gloves when handling the peppers to avoid burning your hands and eyes. Remove the seeds and most of the ribs from the jalapeño peppers to make a sauce of moderate heat, but if you want to breathe fire, feel free to keep the ribs and seeds, and/or use hotter peppers (like serranos or Lumbre peppers).
Throw everything into a high-speed blender (like a Vitamix) or a turbocharged food processor. Purée until smooth. (Yes, a regular food processor will also work—but you’ll want to cut the peppers and garlic into smaller pieces, and blitz the ingredients longer. Otherwise, your sauce may end up on the chunky side.)
Pour the purée into a medium saucepan and bring to a boil over high heat. As soon as it boils, reduce the heat to low and maintain a simmer for 5 to 10 minutes, stirring occasionally.
Cooking the sauce concentrates and deepens the flavors, and cuts the sharpness of the raw garlic. Once the foam subsides, the sauce should be a vibrant red color, and you shouldn’t be able to detect any raw vegetable smell.
Taste and adjust for seasoning if necessary, and then transfer the sriracha to a jar (or three) and allow the sauce to cool. You can keep it in the fridge for up to a week or freeze in ice cube molds for up to 4 months.