How to turn a game idea into a set of tasks. How to develop an idea into a tangible product. Is it 2d,3d, what sort of inputs? What genre, does it support multiplayer, any specific input device. A story can take many forms so how do you image it going and developing into a game to play.  

This is what is called project management. Fundamentally the way you build any project is always the same. Decide what you want, a goal, break it down into its constituent parts, and list features you can verify. Once you’ve collocated the core information about the features, mechanics, game play, the controls, and components in a document this give an overview of the feasibility.  

The famous SMART project planning mentions that a deliverable project needs to stick to 5 key principles  

S for specifics, you need a tangible goal to follow  
M for measurable so you can assess when it’s done or how much is done  
A for attainable, a realistic goal and se of tasks  
R for Realistic, knowing your budget, skills this is to be possible  
T for Time-specific. Set a timeline. Set points where you consider continuing or not, Set point where you can start showing your work.  

The idea being in order to make a game, an any project you need a structure and a way to validate you’re going through all the tasks required to build and develop the final product. You may break it down into milestones.  

For instance let’s first make a prototype to demonstrate the fun and the core mechanics of the game. Grey boxing, albeit very limited, can demonstrate the fun elements of a game. At this point there is a lot of pretend in the prototype and it needs to focus on a limited set of concepts to review.  
Then let’s do a vertical slice, A small playable portion of the final game that gives the actual experience the player will have eventually.  

Then the full blown production once the prototype and proof of concept deem valid and reasonable.  
Use your imagination. Play the game in your head. And have fun.