The rules of blackjack are quite simple. The basic objective of the game is to draw a total closer than the dealer or reach the total of 21. You are given two cards in the beginning and receive additional cards unless you decide that you are close enough to 21 or your total is greater than 21, in which case you lose your wager. After receiving your first two cards, you can play Double Down, which means that you double your bet and receive exactly one card. This is a good strategy in case you have a hand that requires exactly one hit, and you are reliably confident that your hand is better than the dealers.
However, the option to double is often allowed on the players' first two cards only, although some casinos do allow doubling after splitting a pair. For instance, many Northern Nevada casinos allow doubling only with the first two cards. Some casinos do not allow the player to double down on any two cards, and in such cases you should take a hit instead.
You can also use the option of double blackjack while playing in a face down game by tossing the two cards face-up on the table in front of your bet. While choosing a double down option, you can add an additional bet to the betting circle by placing the additional bet adjunct to the original bet, not on top of it. However, in the face down game, the dealer usually tucks the additional card face down under your bet, to be disclosed later.
Players are allowed to double down for any amount up to the original bet amount, so you could also double down for less. However, if the correct play is to double down, you should always double for the full amount if possible.