Here are the very simple steps that I have went through.
1. Get a Pertner that also has good art.
2. Think of plants. If you can't think of many, do you 4/5 plants.
3. Think of zombies. Zombies should be based on your world.
4. Make premiums or gemiums. They don't have to be world suited.
5. Make a world exclusive plant. The top thing you will have to do.
6. Think of the main gimmick for the game.
ALso,,
I will be willing help someone.