The double lift is one of the basic sleights of card magic. It allows the magician to remove two cards from the deck as if only one was being held.
Hold the deck in the left hand as the top drawing shows. The right fingers hold the top edge of the deck lightly and the right thumb rests at the bottom left corner. Tip the deck so you can look down and see the cards under the right thumb.
The right thumb lifts the edges of the top cards allowing all but two cards to fall back onto the deck as the second drawing shows. The right thumb now presses forward on these two cards so they press lightly against the right fingers and the thumb and fingers can lift the two cards free of the rest of thedeck.
If the right thumb and fingers squeeze the two cards slightly, the two will stick together as if they were only one card. There are many ways of doing a double lift. If you are really interested in card magic, you will want to learn at least one other method to do this often used sleight.
There are many ways of using this sleight but this is a way to use it to explain how you do magic with cards.
Shuffle the deck and double lift to show the 2nd card's face. Place both cards back on the deck and take off the top card holding it face down. Say, "If I really don't want the (name the card), I can simply change it for one I want by rubbing the spots through the deck."
As you say this, put the card part way into the deck and rub it a bit before you remove it and show its face. While they look at the card in surprise, cut or shuffle the deck.