Remove the outer fiber from coconut, wash and keep ready (Step wise Pic 1).
Soak rice for 1 hour at least.
Make one hole in the three eyed coconut and remove all the water, reserve it for later use (Step wise Pic 2).
Put Jaggery, Rice, Pottukadalai, Sesame seeds, cardamom powder one by one and a tsp of coconut water at a time (Step wise Pic 3).
Do repeat alternating ingredients but add more jaggery also make sure you add only half of the coconut water.
While inserting the ingredients use tooth pick/small stick to fill inside the coconut.
Once done filling the coconut, wash by closing the hole with your thumb.
You can roast it immediately or do allow to the ingredients to soak in coconut water for a while say 30 minutes to 1 hour so it cooks perfectly.
We usually fill the coconut previous night and on festival day the early morning roast it in fire.
But most people roast the coconut in the evening on Thalai Aadi day.
Before that apply turmeric all over coconut and kumkum(optional).
Now bring it to fire and roast till the whole coconut turns black, otherwise the inner won't cook completely.
Turn all over for even cooking, check the video at the end of the post to gain more ideas.