Place the shelled seeds in a strainer or colander and rinse with cold water to remove any small debris. Pick out any loose shells or large plant matter. Preheat the oven to 300ºF/150ºC. Try not to have any of the seeds overlapping. Roast for 30 to 40 minutes, or until the seeds are brown and crisp. Stir the seeds occasionally to brown evenly on both sides. You can serve the hot sunflower seeds immediately, or leave to cool before storing in an airtight container to enjoy later.  If you like your sunflower seeds salted, sprinkle the seeds with salt while they are still on the baking sheet. You can also mix up to a teaspoon of butter with the hot seeds for an extra tasty treat!

TL;DR in Vietnamese:
Rửa sạch khoảng 1 cốc hạt hướng dương đã tách vỏ. Lót giấy nến vào khay hoặc đĩa nướng. Dàn trải hạt hướng dương trên khay nướng thành một lớp mỏng. Cho hạt hướng dương vào lò nướng. Thưởng thức và bảo quản.