Honestly, there's nothing that you can do to prepare for this? If you know how to play the game itself, that's fine; however, if you can think you can make a broken card because you want to "win", then stop reading and move on to the next post, please, because that's what this post is NOT about. Now, to settle some basic statements:
1) There's no set boundary to what you can make. You can make whatever your heart desires, but be warned, because if it doesn't work well enough or if it's too requirement heavy, you'll never see the use of that card.
2) However, in some cases, you need to focus on two things: "Broken by design vs. Broken on how it is played".
Typically, I would go through each way to push for creativity, but in this case, not exactly. If you push for something that they have no understanding of, then there's no point in them pushing for more knowledge. So I decide to push for one final look and decided to give them a basic point on where some cards lie.
Broken by design:
Typically, these cards would be rejected on how they are seen. For example, you don't want a card that blows up your opponent's cards each turn. For example:
Destruction of Hope
[Continuous Spell Card]
During each Phase of your opponent's turn, destroy all cards on your opponent's side of the Field. This card cannot be removed from the Field by your opponent's card effects.
Pretty self-explanatory, this card is broken beyond belief and whoever made this card needs to stop trying. If you start off with this as your basis, then don't bother making cards, because no one wants to see One Turn Kills (OTKs) run loose.
Broken on how it is played:
This one is the more fun of the two. It shows that you know how it works, AND the cards themselves are usable to the point of where people want to play them for what they are. However, it can get pointless making these types of cards, because if everyone breaks it, then it'll soon be rejected. I won't give an example of this, because there is no set example. You need to figure this one out, on your own.
Well I explained the basis of both of those, and basically told you that there's no set path you can take while making these. Regardless of what you think, I still want people to try and make some cards, because:
1) It is fun.
2) It is a perfect way to enjoy yourself while waiting for an appointment or if you have some free time.
In any case, it doesn't matter what you decide to do. The choice is yours.