This question can be difficult to answer as the rules for card splitting vary from game to game. In general, however, you can split cards if your hand totals are at least 21 and the dealer has a 10 or face card.
If the dealer has a 7 or blackjack, you cannot split cards. .
One rule that is common to all card games is that you must declare your hand before splitting cards. This means that you must say out loud how many cards you are splitting and what card you are splitting them into.
Once you have declared your hand, the other player must either accept or refuse your offer. If they refuse, then your hand is unchanged and the next player takes their turn.
If they accept, then both players put their original cards back into their hands and the next player takes their turn. The player who splits the cards first takes the advantage of having two extra cards in their hand.
This gives them an increased chance of hitting a higher number in future rounds of play.