First, I look at what needs to be designed; If it is a Game Mechanic, I will look at what the games themes are and then design the mechanic with that in mind. I also decide what would be an exciting game loop to design. Then write a detailed Game document about my ideas.
The next step after my Game Design Document is completed is to create parts of the idea in a prototype. Once the idea is implemented, I look into having people and me playtest the game mechanic. The playtest also needs to be done with the game's target audience.
After the playtesting and receiving player feedback on the mechanic. I would look at this feedback with a team, then decide what feedback we want to take into account and make those changes to the Game Design Document.