I still haven't understood, how was the poison mixed with Joffrey's cake? Is it explained in the show yet? I have a feeling, I have missed the explanation. This much I have understood that, Little finger put the poison in Sansa's necklace which somehow mixed with the cake. But when and how? Please help me understand it.
In the show, it is not explicitly explained how Joffrey was poisoned. Whether it was the wine or the cake.
Littlefinger & Olenna were directly involved. Olenna confessed to it in front of Margery in S04E04. Ser Dontos was hired to transfer the poison onto the unsuspecting Sansa.
If you go back to S04E03, you can clearly see the point at which, Olenna takes the poison from Sansa's necklace & puts it in Joffrey's wine.