This is a tutorial on a simple, easy magic trick, involving a setup known as the "slug".
Step 1: Setup the "Slug"
In this step, you must make a setup called a "slug". This slug must be identifiable; that is, you must be able to realize if there is a unknown card in the slug. For this example, I used all of the face cards of a deck as my slug. To start, I put all of the face cards together to form the slug.
Step 2: Put the Slug Ontop of Your Deck
Step 3: Control the Deck
..And this, is where everything can go wrong.
Basically, you now want to manipulate the deck so that a spectator chooses a card outside of the slug, and puts it back into the slug. To achieve this, you want to manipulate where your hand positioning is.
If you want the spectator to choose a card not in your slug, keep a sturdy grip on the slug part of the deck, then "fan" or "loosen" the other cards out towards the spectator. Chances are, the spectator will choose one of the fanned or loosened cards, than the sturdy ones gripped tightly in your hand.
The same thing goes when you are putting the card back into the deck. Manipulate the deck so that the fanned cards or loosened cards are now the slug.
NOTE/TIP: The more cards you have as a slug, the better chance that the second part, where the spectator puts their card back, works successfully.
Step 4: Ruffle Through the Deck, Look at the Slug
If you notice, you will probably see the 9 of hearts in the image provided. Because it is not a face card (and the slug was made out of face cards only), then it is the spectator's card!
NOTE/TIP: Once you have identified the spectator's card, you can memorize it and then shuffle the deck, cut the deck, or do what ever... you know which card the spectator chose.