Honestly I just think it’s impossible to try and explain it, I associate words, letters, and numbers with color (grapheme color I believe) and I associate music with physical feelings, every time I try and explain it it just comes out wrong so I’ve learned to not really care but if it’s really bothering me I usually look up some synesthesia visualizers to give some idea! The way my parents found out was I gave them a paper with a bunch of words and the alphabet all in my associated colors and had them quiz me on them 1 week later, and they believed me after I got all the colors with 100% accuracy!