Be assured playing the Svengali deck card trick is a technique that requires clean execution of the trick and its finishing. The audience at no stage should come to know that you have played a trick on them. If you are exposed, the entire surprise and enjoyment of the trick is wasted, and you are likely to get boos from your audience.
Start with finding a person from your audience to sign a duplicate card, just before you enter the arena of your show. Place that card in an impossible location. It can be your wallet for example, or any other place that is not possible for the audience to locate.
Before you announce for your audience to gather and take their seats, set up deck of your card by mixing ordinary with duplicate cards. Then riffle shuffle through the cards by claiming you are mixing them. Actually the cards fall in pairs and hence are not mixed, while audience apparently realizes that they are being mixed.
Go on then to riffle through the deck, and ask a member of the audience to help you to stop the riffling whenever they want. After stopping the riffling, ask the audience member to pull a card from the top, with face down.
When the member of the audience pulls the card, write down the name of person that you have earlier written down on a duplicate card, just before entering the venue. Show the card with the name written on it to everyone in the audience.
After that ask the member of the audience to put the card back in the deck and riffle through the deck again slow. After riffling announce to the audience that card is missing, and show all the cards to them, if they demand. Since the cards fall in pairs, the card with name written on will not be at front but an ordinary card.
Now pull the card from the impossible location and show it to your audience. This card is in fact the one on which the member of audience wrote, while actual card is still in the deck but paired with an ordinary card.