It is sometimes known as Solfeggio, when using do re mi fa, etc. But singing the parts is just part of practice. To start to do that, sing along with something you know. I use the count at first, with the number sung at the pitch of the part I am singing. If your range does not allow this, then sing an octave higher or lower. Then, I will sing the letter names to help with memorizing the piece.
Since you have probably been using your voice longer than playing guitar,
you will be able to convey something
when you sing the part. Listen carefully to what you are singing. Then play
what you just sang.
Singing will help you understand phrasing, too.
I found that taking a few vocal lessons was useful.
When the sun shines, bask.
Classical Guitar forever!