In craps, there are two "Phases". 1) Before the point and 2) with the point established.
On the first roll, if it is a 7 or 11, the bet on the pass line wins automatically (This is where you want to get 7 twice in a row)
If it is a 2, 3 or 12, the bet loses automatically. (But then you have another opportunity to roll your 7s)
If it is any other number, the point is established, and you will notice an "on" chip placed above the same number as the opening roll.
With the point established, you cannot make progress towards this trophy until the next "Opening roll"
With the point established, play will continue until a 7 is rolled, which will make the pass line bet lose and start the game back at the opening roll. Or, if the point number (the one with the "on" chip above it) is rolled, the pass line will win and end the game, starting it over.
Note that if you end the game by rolling a 7, this does not count, only 7s rolled on the opening bet count.
Now that the game has ended and you're back at the opening roll, you have another chance to roll 7 twice. As you can see, it's possible, depending on how the game ended, that you will have to roll a 7 3 times in a row.
There are many more rules to craps, but these are the rules important to this trophy. You can bet anywhere on the table while going for the trophy, you just need two 7s rolled in a row during the opening roll (Or, before the point is established, without an "on" chip present on the table)
To summarize, when there's no "on" chip, this is when you want 2 7s in a row, you can ignore everything else and play randomly if you want and just pay attention to whether or not the "on" chip is present. If the chip is present, just keep rolling until it goes away so you can try again. This is a pure luck trophy.