She just did it so fast, like a cheap magician, that it's barely noticeable in real time (just the smallest hesitation in the movement of her arm going down, as she teared the stone away), but you clearly see her snatching the stone/vial if you freeze the right frame (linked picture with the stone in Olenna's fingers).

The necklace with a missing stone is clearly visible on the next shot of Sansa (no, it's not hidden behind her hair, it's really gone). Olenna took off the last stone on Sansa's left side.

http://i.kinja-img.com/gawker-media/image/upload/s--0tsTwAPq--/c_fit,fl_progressive,w_636/pyznszcs3rxc9gofzul8.png

(sorry, that picture isn't as obvious as some others I've seen where the frame had been scaled up and you clearly see the empty socket, but it's the best I could find with Google.)

For sure this wasn't supposed to be noticed by 99% of the non-readers audience. It's only when Sansa notices one stone is gone that more people are going to remember that Olenna's the only one who fussed with Sansa's hair and necklace. But the production didn't cheat, they really had Diana Rigg do it on screen.

As for when she slipped the poison in the goblet, we don't see it happen on screen, but notice Olenna's focus of attention through the scene. Her plan was most likely to put it in Mace's chalice for the toast which was supposed to happen after the "main entertainment" (diverting suspicions because Margaery would have been expected to also drink from that chalice after Joffrey, supposedly after Joffrey said a few words after drinking, still leaving little time for the poison to act, and to stop Margaery from drinking), but Joffrey was going off program and Olenna seized the better opportunity of putting it in the goblet Sansa and Tyrion had just handled in front of everybody. Sansa in particular has given Olenna a golden opportunity by fetching the goblet from under the table.

IMO, Olenna did the deed while all attention was on Joffrey's antics. When she diverts Joffrey's attention from Tyrion to the pie, Margaery took his cup and placed it on her far right corner of the couple's table, closest to Olenna's place (but still not on the same table - and a few feet away), and the two exchanged a look. Then Olenna rose to applaud the release of the doves, and it could be when she managed to pour the poison, unless she had passed the poison to Margarery, who actually did it (next week's preview suggests that maybe Marge didn't know of the plan to murder Joffrey but... we'll see in context if she really says what we think she's saying... I think she might have done the deed without knowing that the substance given her by Olanna was poison). In any case, Tyrion had to come fetch the cup all the way on the other side of the couple's table, and for sure between the time Margaery took it from Tyrion and the time Tyrion grabbed it from the table, it's been poisoned by either Margaery or Olenna.

Martin and the director pulled it off quite well, I think. Part of the changes were there to help fool the audience and give them a bit of a whodunnit for one week. Martin is flexible and adapted quite well to the different codes used by the TV show.

Misinterpreting their body language and exchanges of looks, part of the non-readers already convinced themselves that Sansa in a suicidal move used nightshade procured her by Tyrion earlier, poured it in the goblet, which Tyrion saw, and in which he poured wine - and that Ser Dontos saw her do it and came to rescue her. Not sure what convinced them Sansa would be capable of a move like that, or why she prepared for an opportunity she was so unlikely to get, but oh well.

A whole lot of the rest seem convinced that Cirsei or Tywin or both scheming together had it done, through the pie instead of the wine. They think either stand to gain the most from the murder, having Tommen in reserve to offer the Tyrells, which isn't illogical. Tywin would be able to accuse Jaime's Kingsguard of failure and would have his excuse to get Tommen to expel Jaime, and send him to the Rock. Cirsei could believe that Tommen is too young for her to leave King's Landing and this might get her rid of Jaime (non readers have bought her act that she no longer cares for him...) while others believe she made a deal with Tywin to get out of her wedding to Loras, by putting Tommen on the throne, work to send Jaime to the Rock and get rid of Tyrion all at once. Not how the books's Lannisters would have thought, and not consistent with the show's Cirsei who will not doubt be doubly horrified that her sweet Tommen will be given by Tywin to Margaery, but oh well... (I suspect Cirsei's revenge on the show could involve finding a way for Ser Loras to enter the Kingsguard).

Some do suspect Olenna or Mace, but following the same logic many of the characters would follow, they doubt it as they don't see the Tyrells' interest in losing so much money and political efforts invested in that alliance, on the off-chance Tywin will agree to substitute the wedding to Joffrey to one between Margaery and Tommen, who can't even get Margaery pregnant with a son for a few more years.. and thus constitutes a far less solid alliance for the Tyrells. And if they wanted Joffrey dead, a quite reasonable assumption they would have most likely waited until Margaery bore a son from Joffrey. That explains why the Tyrells knew they wouldn't be suspected too much, but the audience underestimate the political daring, skills, and the military and financial leverage on the Lannisters until the war is truly over and paid for that Olenna possesses, which pretty much guaranteed Tywin would offer Tommen to Margaery as the only way to save the alliance.