Once you have some good ideas coming out of your brainstorming, then you need to start prototyping.
Prototyping is the stage where you make a quick-and-dirty version of your game to see how and whether or not it actually works.
Don’t get too invested in only one idea. Make various quick-and-dirty prototypes.
And don’t worry about making the prototypes the best version of game possible. Make it quick!
My game design process subscribes to what is called rapid prototyping in which you get to a prototype of your game as quickly as you can to see how it works. Then you test it, find the problems, and make improvements through iteration.
And don’t wait for a better way to build your prototype. Build it now!
If you’re making a video game and you can’t code, for example, that doesn’t mean you can’t make a prototype.
Make a paper prototype.
Use some dice or cards.
Whatever you have on hand is good enough to make a prototype.
This is an early stage of experimentation. And you need to make many rapid prototypes.