During preparation for practice I try to internalize what parts of the music I want to play. Technical aspects like fingering, and how the rhythm is counted are quite straightforward to prepare and get reinforced before I play, however I've never been able to actually hear the music in my head as I read it - I have to play it first in order to hear what it sounds like - it feels that I am relying on my eyes to identify the pitch as opposed to my ear, and am almost always to trying to find recordings of the practice pieces in order to contextualize what I want to practice (which isn't a problem in itself.) It just doesn't feel like a intuitive skill for me and was wondering how I could develop it.
Different people do it differently, but the way it works for me is, I "play" the notes in my head, and it makes me "hear" it. I've developed this skill through playing live by ear and improvising, following and tracking what other instruments play at the same time as they're playing it. Through lots of practice, the functions of hearing and playing have fused into one. Hearing is playing, playing is hearing. If I want to hear it in my head, I play it in my head. There are probably people who can hear what they see, but I have to play it in my mind first. When I read music, I'm tracking the notes I see on a keyboard in my mind, imagining I'm playing the keys. And then it's almost like hearing it. Though what I "hear" might be wrong, it's not 100 % accurate. The more complicated the music is, the lower my guess hit ratio is, and for complex harmony changes I have to actually play it.
You should look up everything written about "solfege" (questions and answers) in this SE and practice singing all the intervals, writing tones, of writing memorized music. Very helpful could be singing and writing canons in 3 or 4 voices singing, whistling or thinking all the other parts. So you have to teach all voices and assemble them all together.
If you are able to notate the music you hear with your inner ear you will be able to hear with your inner ear the notated music.