Two cards dealt face-up to each player. In order, each player bets from nothing to however much is in the pot whether or not the third card dealt face-up will numerically fall in between the first two. If the player wins the bet, that player collects the bet amount out of the pot. If the player loses the bet, that player adds the bet amount into the pot.
The dealer deals one card face-up to each player including him or herself. The dealer then deals a second card face-up to the player to the left. Before any more cards are dealt, that player bets whether or not the next card off the top of the deck will numerically fall in between the first two cards. The player does not need to bet.
If a bet is made, that bet will be from the table's minimum denomination bet to the entire amount in the pot. After the bet is made, the dealer throws face-up the next card off the top of the deck. If, for example, the player is dealt a 2 and a 10, and the next card off the top of the deck is an 8, that player collects the bet amount from the pot.
If, for example, the next card is a jack, that player pays the bet amount into the pot. Once the player has either declined betting, won, or lost, the dealer then deals a second card to the second player to the left and the same options apply. This sequence continues around the table, including the dealer who has the same options.
If the third card dealt to a player is the same as one of the first two -for example, the player has a 2 and a 10, and the next card is another 10- then that player has not only lost the bet, but has "hit the post". This player must pay double the bet amount into the pot. If, for example, the player bets everything that is in the pot and "hits the post", that player must pay into the pot double the amount that is already in there, thus tripling the size of the pot.
When this game is called, the first thing that the dealer needs to specify is how long the game will last. Because cards are not reshuffled until the game has gone through the entire deck, the dealer may decide that the game will go through the deck a certain number of times ("Game ends when we've gone through the deck three times..."). Or, the dealer may decide that the game ends when a player bets pot and wins, thus emptying the pot. I have always found that the best formula is a combination of both principles.
A good way to start, for example, is to specify that the game will go through the deck at least twice, after which the game will continue until the pot has been cleared out. In this way, if the pot is emptied before the game has finished going through the deck, then every player (including the winner) simply re-antes and the game continues. Another way to prolong the length of the game is to call that players can only bet a maximum of half-pot until the game has gone through the full deck once.
This all depends on the table and what everybody is willing to put up to start. The bottom line is that the ante in In-Between should be significantly higher than the normal game ante. A good formula is to say that the ante for In-Between be three times what the usual ante is. If you are playing at a knickel-ante table, then ante for In-Between would be 15 cents. If regular ante is a quarter, then ante for In-Between would be 75 cents. Bear in mind as well that if the dealer decides everybody must re-ante if the pot is emptied before the game is finished, that this same amount will need to be re-anteed by each player.