Place the annatto seeds in a medium nonstick skillet and add 200 mL of cooking oil.
Turn the heat to medium and stir continuously. Avoid using high heat, as this can cause the seeds to burn and the oil to darken.
Once the oil begins to bubble, lower the heat and continue stirring for an additional 3 minutes.
Remove the skillet from the heat and strain the oil into a bowl.
Return the skillet to the stove, add back the seeds and then the remaining 100 mL of oil, and heat over medium-low. Stir gently.
When the oil starts to bubble again, reduce the heat and stir for another 3 minutes. Add the garlic at this stage.
Remove the skillet from the heat and strain the oil into the same bowl as the first batch.
Allow the oil to cool completely, then transfer it to a glass jar for storage. Your annatto oil is now ready for use.