How do you make your magic system understandable?

Currently making a fantasy story (and it's going terrible as I suck at world and magic building) and one of the things that I am spending most my time on is the magic system. <br>I want to make a magic system that is realistic and has it's own rules that I need to stick to, and I was wondering on how to do it.<br><br> <br>I see how magic systems are woven into the lore of other fantasy stories like Harry Potter, The Witcher, Dragon Age, and others but I don't know on how to do it properly, so any help on it would be great.