Pick out the impurities from the seeds and then soak with clean water in order to remove the floating ones.
Wash under running water over a sifter or something similar and then drain
Heat up a pan over medium fire, place the sesame seeds in; slow the fire to lowest heat and keep stirring continually until you can smell the aroma and the seeds are slightly popping. It may take around 3-5 minutes for one cup of sesame seeds.
Transfer out to a larger pan to cool directly, otherwise the heat of the pan might burn them and further resulting a bitter taste.