How do you make a d20 game?

To create a d20 game, start by establishing the game's setting, genre, and core mechanics. Develop a character creation system that allows players to choose classes, abilities, and backgrounds, using a d20 for skill checks and combat resolution. Create a rules framework for gameplay, including how to handle combat, skill checks, and progression. Finally, consider crafting a campaign or adventure to guide the players, ensuring to balance challenges and rewards throughout their journey.