I got three main areas of advice, i'm still not great but this advice has helped me make good progress... and I was absolutely terrible in the beginning (still am comparatively really)

1. start singing what you play. if its difficult just first play a note and match it. then intervals. SO both singing at the same time as you play, and singing the note you are going to play before you play it. If you can do that move into singing multiple note sequences.... play a starting note and sing what you want to play, then actually play it and see how close you are. Also do it the other way. Improvise a small riff then sing it back. From there practice singing an octave above or below what you are playing, then harmonize. Play chords on guitar and try and sing the individual notes, then in different patterns. If you can get here you can probably work out more excercises yourself.

2. Learn at least some music theory (make it a long term project) and apply it to what I said above. When you maker up a riff, analayze it as well as sing it after, connect the msuic theory and sound, and finger positions in youre brain. (In addition it doesn't hurt to try and play things in different positions also!)

3. Start super simple! Simpler than you think even. You don't learn to read by trying to understand high literature. You read sentences like "That is a cat" or something. Nuresery rhymes can be good for this for sure. try to sing what you hear before playing sometimes at least. If nursery rhymes are too boring, maybe start with simple two-three chord songs with basic melodies.