This one is a little tricky, but doable.
1). Select an Expert puzzle.
2). Once the puzzle opens, look at the numbers below the puzzle. It shows you specifically how many of each number is available to place on the puzzle's board. Make sure that the number seven displays a smaller seven above it, indicating that there are seven sevens available for placement on the board that you are playing.
3). Select 'Menu' to the left of the puzzle. If there are not seven sevens available to play, select 'New Game' until you start a new puzzle that does have. Once a seven-sevens puzzle is started, open the Menu again.
4). Select 'Show All Notes' from the Menu.
5). Select all the "cheats" to ease the gameplay. Block Duplicates, Show Incorrect, and Show Guides.
5). Use the tiny numbers that are now displayed on the board (Show All Notes) and the "cheats" as a guide to help you select every number other than seven within the board. Make sure that you do not type seven!
6). When all numbers have been placed except for seven (or more) sevens, place all of your sevens, finish the game, and watch your Achievement "bing"!