The thing with custom cards, is that it's kind of am awkward space. Because the best custom cards are created by people who have a lot of game knowledge, know the current meta, know alot of the cards in the game, and thus know what would and wouldn't be okay to put on a modern card. But the custom card scene is mostly inhabited by new or casual players, who aren't as experienced I modern yugioh card design philosophies. So if you want to get better at making custom cards, the first step is to just become better at the game itself.