There are many patterns that could win you a bingo. If the particular game you are playing stipulates that you need to have a "diagonal bingo" in order to win, you must form a diagonal line across your bingo card, which includes the middle (often known as "Free") square.
So, as the bingo balls are being called out, watch for a diagonal line going from either the top left corner across to the bottom right corner, or, a diagonal line going from the bottom left corner across to the top right corner on your bingo card!
All diagonal bingos use the bingo free space, so it is a common way for players to bingo. Just call the Bingo Spotter over when you think you've got one!