As soon as you get the clams, transfer them into a plastic bowl, cover it and place the bowl into the deep freezer
Next day, remove the clams from the freezer and let it come to room temperature. You will notice that the shells will open up. Add 1.5 cups of water to the shells
Split open the shell and transfer the fleshy side of the shell into another bowl and discard the empty side
Now Tesreos have to be washed in the same water(that's what I was taught in Goa by my neighbour aunty Rosy) thrice or until all the 'czherr czherr' dirt come out. Basically the fine stones/sand/mud and particles alike. So now you have a bowl with the Tesreos and a Bowl with the water.
Gently, pour the water in such a manner that you don't disturb the bottom of the water(that's where all the dirt settles down) into your Tesreo bowl and discard all the dirt particles that remain, by washing the bowl.
Gently wash each shell in the same water, and remove any excess dirt/fine particle and transfer the cleaned shell into the empty bowl.
Repeat steps 1 to 4 till you feel the water is completely free from any dirt particlesđ It takes about 4-5 times to wash the tesreos
Note:- If you're comfortable breaking open the tesreos on the Traditional Coconut scraper(Kaanthne/Adavo), you can do that too. In that case you can skip the freezer step.