If you're concerned about sodium, soak olives in water overnight and drain. Pit olives if they are not already pitted.
Place the dried chiles in a bowl and pour enough boiling water over them to completely submerge them. Allow the chiles to soften (about 20 minutes). Drain, reserving some of the liquid.
Toast the cumin and coriander seeds on a dry pan over medium low heat until fragrant. Grind using a spice grinder or mortar and pestle.
Place drained/pitted the olives, drained chiles, garlic, and ground spices into a food processor. Process to desired consistency. Add the reserved liquid if you desire a thinner consistency.
Note: This is a general template for harissa. Other harissa recipes call for roasted bell peppers, mint, onions, and other spices (like caraway seeds). As part of my restrictive diet poker, I eliminated the olive oil and added salt by replacing it with whole olives. While olives are soaked in brine, I attempted to mitigate the sodium level by soaking them in water before hand.