Fusion is confusing

I have played Persona 3 for around 25 hours/ I’m at mid January and I find the fusion system difficult to understand. I want to learn how to fuse efficiently and to get better Persona’s. This is also because I chose hard mode and have struggling severely on the full moon bosses. The only Persona I fused for was Jack Frost. If someone could help understand how to fuse well and how to get stronger Persona’s it’d be a strong help.