A digraph is formed by two letters but has only one sound.
E.g. PH / f/.
If the 2 letters are vowels don't forget that :
"When two vowels are walking , the first ones does the talking"
( Cuando van dos vocales juntas , la primera dice su nombre y la segunda se calla, no suena.)
E.g. UE /juː/
Here you have DIGRAPHS SONGS.
Listen to the video and practice the sound of each digraph.