This'll be the easiest to remember since there's a rule of thumb on this: "Don't make it super overpowered broken." For example, this card:
A rule of thumb is: "There's a
fine line between having a good card that would be playable in most
decks vs. having it broken and in every single deck." All you have to do
is take some time to practice on making cards.
Next post will be "Yu-gi-oh! Create-a-Card Lesson 2 - Broken on How it is Played".