You can only do this on a horizontal row with no more than one letter in it. What I did was look at my hand and then look for a single letter that would not, when creating a word, cause me to bump into other letters. For example, in my game below, the letters, E, V, Y and S are all alone and allow me to create a clean left-to-right word.
Also note, it looks like some rounds indicate that there's a bingo word. I noticed this today, but I did not get the bingo! The text at the top appears on the game board, and at the end the longest word is probably the bingo word :)