Cut off the stems of the Guajillo peppers. Shake out the seeds, you can also cut them in half along their seams and split them open to get the seeds out as well.
Cover the peppers in boiling water for 10 - 15 minutes to soften them. While they soak, add the rest of the ingredients (except the pepper water) to a blender.
Once peppers are softened, add them to the blender as well and blend until smooth. You can add a few tablespoons of the pepper water if it's looking too thick (I usually add 3 - 4 Tbsp worth)
When Al Pastor marinade is blended, you can either marinade the soy curls in it for later use, or simply pour over them after you've already browned them. I personally l think it's better to pour over after browning the soy curls.