Digrams are tiles that have two letters on them, like "er" or "th." Find any word using one of these tiles.
If you can't find a digram word organically, try randomly entering words using the digram. Try each possible word that starts with the digram and ends with one of the 3, 5, or 8 letters around the digram (depending on if the digram is in the corner, on the side, or in the middle, respectively). If you have no luck there, do the opposite and try each possible word that begins with a neighboring tile and ends with the digram.