Simple bullet point guide to game design. This is a loop so when you are tired of the game or run out of money exit the loop and ship it.
Step 1: Ideas
First get a cool idea for a feature or change in your game.
Gather up all ideas from members of the team.
Make a suggestion page on the inter web where anyone can leave suggestions.
Step 2: Design
Take a possible feature and check them against the current design and see if it fits.
Check possible side effects. How it will affect the rest of the design or if it will break any other game feature.
The more time you spend making sure the feature really looks like it could work the less risk that the rest of the team waste time on something that does not work in the end.
Step 3: Document
You document the game's design so you do not have to keep everything in your head and to make it easy to work on it as a team of designers.
No one besides the design team will ever read more then a paragraph in the document even if you hold a gun to their head.
No really, do not waste time to trying to change that.
At the top of a subject write the most important facts of how things will be as bullet points and with pretty pictures. This is what the rest of the team needs to know.
In the lower part keep the design work materials such as why a design is like it is, why some other feature was not choosen and so on. Things you need in the future.