To get this achievement you have to select Bridge Builders as your game type in the settings menu, and start up a game. You play words just like you would in a regular game of scrabble, with the exception that you can't build off any word, you can only build off YOUR played words. You start on the left hand side of the board, building horizontally then vertically, and keep flip flopping until you reach a space on the right edge of the board. Only one letter has to touch. Finish the game, and the achievement is yours.