There are a few good places that give you a guide for this. My advice is simple for cards.
Keep it simple, include a status quo, conflict and resolution.
The main issue I find people have is that they try to 'tease'
"David had an amazing secret that the government want to get hold of, can he escape with his life?"
That does't work for us here, it's a sales pitch, what we need is something like
"David discovers who killed JFK, the government pursue him to stop him revealing it. He manages to release the information and escapes into hiding"
This is a description of what happens, we can work with this.
So go on -try it!