Game Prototyping is a skill on its own, not so much the prototyping part, but more the “focus” on the gameplay mechanics, and fun, instead of graphics, or theme.

Create a prototype that solves a specific purpose, for example, if you want to do a prototype on the combat mechanics in your game, focus on that, and only that.

Types of prototyping:

Game Prototyping
Environment Prototyping

Coreloop Prototyping

Game Prototyping

Full game prototyping, from start to finish.

Game levels
Art and Audio
Character controllers
Gameplay mechanics
UI

Environment Prototyping

White-boxing, also known as grey-boxing.
2D Layouts
Play with level ideas
Make multiple attempts
Get feedback Early
Revise Quickly
Tools for creating top-down layouts:
Photoshop
MS Paint
Paper and Pen
Google Sheets - yes, even game layouts
Draw.io
Start with the players starting position
Player’s starting position
Create a legend
Trees, Buildings, Gold, Enemies, Water, Bridge
Shows where enemies spawn
Show enemy patrol paths
Show player movement path
Blocking
Whiteboxing
Creating a first-pass of a level
Using only white cubes and other primitives
Cubes, spheres, cylinders, capsules, planes
Scale your 2D reference to the primitives in the Game Engine. For example, the cube in Unity is 1m3 (1x1x1).
Get a sense of the space
Create prefabs as soon as possible, as it would make it easier to replace later.
Creating Prefabs
Placing Prefabs
Level Flow
Test and Repeat

References

[DOOM] John Romero’s Level Design Rules: https://www.youtube.com/watch?v=ptHurafdCoQ
The mistake every new game developer makes