Guidelines (Only the first one is an actual rule.)
1. Let the cards speak for themselves. If their is important context for the cards,find a way to fit it into the flavor text.
2. Submit as many cards as you like. The more the better!
3. I usually like cards with a power level that doesn't exceed modern playable. I also think that creating interesting commons is an underrated art. Your cards don't have to be powerful, simply interesting.