The Double Flop Hold'em game starts in basically the same way the normal Hold'em game does, with each player being dealt two cards to start with and then the pre-flop betting round commencing as normal. Depending on whether you are playing fixed limit, spread limit, pot limit or no limit the rules of betting will be different, but the general idea of the betting round and what precedes it is exactly the same in all of the different versions of Hold'em whether they are double-flop or not.
Once the pre-flop betting round has been resolved, six cards are placed on the table face-up in two rows of three. These are the start of the two boards. Players play hands on both boards and if they feel that one of the boards (or both) is looking nice to them, they can choose to continue with the hand. Once the flop betting round has been resolved, two turn cards are added to the two flops and once the turn card betting round has been resolved the river card is dealt to both flops. One more betting round occurs and then the showdown happens. In the showdown, your goal is to make the best five card poker hand twice; in other words, you treat each board individually and make the best hand you can from the five cards in the board and the two in your hand. Half the pot goes to the winner from the first board and half the pot goes to the winner from the second.
Double Flop Hold'em poker is a very good game for people that want to cut down on variance a bit. While pocket aces might have a tough time and get cut down pretty frequently when it comes to a single flop game, if you are playing with two boards instead of one board then you are going to have a much easier time in making sure that you win at least half the pot. The second board also helps greatly in helping you determine what your opponents do not have, since if you have a King high flush and the ace of that same suit rivers on the other board, you are pretty much guaranteed at least half of the pot. If you've ever seen a game of High Stakes Poker where the players run hands more than once, then you will understand that a lot of people absolutely love poker variations like this that reduce the variance of the overall hand.