In a dry pan over medium heat, lightly toast the annatto seeds, cumin seeds, coriander seeds, and allspice berries until fragrant, about 2-3 minutes.
Remove the pan from heat and let the ingredients cool completely.
In a spice grinder or using a mortar and pestle, grind the toasted ingredients into a fine powder.
In a mixing bowl, combine the ground ingredients with the dried Mexican oregano, black peppercorns, dried thyme, ground cinnamon, ground cloves, ground nutmeg, and ground turmeric.
Mix the ingredients together until evenly combined.
Store the homemade achiote seasoning in an airtight container in a cool, dry place for up to 6 months.