Prototyping is probably the most important step in game development. It determines the overall shape of what the game will be when it completes itself.
A common question being cast by an indie developer is, "How shall we start prototyping our game?".
There are many ways to do it, but the method I recommend is to begin by creating a generic tool for developing the game, rather than trying to come up with a viable piece of entertainment right away. Start by making your own game engine, instead of struggling to draw an outline of the final product from scratch.
This is indeed a tedious process, but also a solid plan when it comes to long-term operational sustainability.
If you start designing a "game" right away, you will be spending considerable amounts of time putting it in the context of specific narratives, themes, mechanics, and other details which are prone to change as the course of development progresses over time, in which case a huge portion of your effort will be wasted.
This is the reason why you should develop the tool first. Implementing features that are rather general than specific will lay the foundation of future design endeavors.