Don’t write games on paper until they are perfect, build prototypes early.
The more times you test and improve your design, the better the game will be.

• Establish a problem you’re trying to solve.
  What does your game aim to do?
• Find a solution.
• Prototype it.
• Rince.
• Evaluate.
  What are the results, risks, and values?
• Repeat or stop.

Some questions you should be able to answer if you somewhat have a direction.

• What is your project?
• What are some inspirations?
• What are some similar projects or competitors?
• What will your game do different from them?
• Who are you working with? What is your budget?

When you try to add a mechanic or feature:

• Why do you want to implement it?
• Who is gonna benefit from it the most?
• In which situations will it be active?
• How the project would be improved by it?
• How simple is it to explain?

Filter them out if the answers to these questions are not satisfying.

Then try to evaluate it (each component is noted from 1 to 10):

• Effort = Time + Risk + Cost.
• Value = Rayonnance + Usability + Depth.

Start with Concepts: genres, philosophies, intents, … Select a handful, describe them with a sentence explaining how the project is influenced by them.

Find the Aesthetics you’re aiming at:

• Sensation, sense-pleasure.
• Fantasy, make-believe.
• Narrative, drama.
• Challenge, obstacle course.
• Fellowship, social framework.
• Discovery, uncharted territory.
• Expression, self-discovery.
• Submission, pastime.

Finally, establish your Pillars. They should be drives or over-arching parts linking Concepts and Aesthetics.