It's a pretty easy achievement to get, even if you don't go for it, but since the achievement explanation doesn't really explain what a digram word is...
Sometimes in a game (not always), there will be a tile with two letters on it instead of one. Which letters depends on what language you're playing in. If you're in a game that has such a tile, simply make a valid word using the tile and the achievement unlocks.
It doesn't matter where in the word the two letters are, you don't have to start the word with them.