Identify your use case (marinade/sauce vs baking) and the role achiote paste plays: color, earthy spice, and thickness.
Choose the closest match from the substitute list: annatto-based for the most accurate flavor; red pepper/tomato/paprika for body and color; harissa only if you can handle extra heat.
Use the listed ratio for the amount of achiote paste your recipe calls for, then blend in oil (or with the recipe’s existing wet ingredients) until smooth.
If your substitute is bitter/too sharp (common with peppers or strong harissa), reduce it with a spoonful of oil or tomato paste; if it’s too mild, add a pinch of paprika or extra acid.
Taste and fine-tune after 2–5 minutes of simmering (for cooked sauces) or after the substitute hydrates in wet ingredients (for marinades and batters).