Cover the dried chilies with boiling water. Let soak for 30 minutes.
Toast the spices in a dry skillet over low-medium heat, shaking or stirring to prevent burning. When the spices are fragrant, remove from the heat.
Grind the spices in a mortar and pestle, spice grinder, or coffee grinder. I use my stick blender with the spices at the bottom of a narrow bottle.
Drain the chilies, reserving the liquid.
Remove and discard the stems from the chilies. Remove the seeds as well if you'd like to keep the heat levels down. Wear gloves.
Combine the chilies, spices, garlic, and salt in the bowl of a food processor. I use my stick blender with the ingredients in the bottom of a narrow jar.
Drizzle in the olive oil while the food processor / blender running and process to form a smooth and thick paste. Scrape down the sides of the bowl occasionally. If a thinner paste is desired, add some of the chili liquid until the paste is at your desired thickness
The flavor of the harissa won't be as good right now. It will have sharpish edges. But you can taste it and make any corrections needed now if you like and also add any optional ingredients. Refrigerate over night, then taste again and make any additional corrections as desired.
Put the harissa in a jar and cover the surface with a thin layer of olive oil. Cover and refrigerate for up to a month. Add additional olive oil to the top as needed to keep it covered.