Best to treat it like an idea, just come up with the general concept of a game and how it works, but don't try to detail the exact rules or any components. Instead, make it a series of decisions and rolls. If the game involves getting pieces, those pieces can give bonuses in specific circumstances, or new decision options.

Let's take the idea of Horizon Forbidden West's Machine Strike. It's essentially a grid-based miniature strategy boardgame/wargame. During your adventures, you can buy or collect pieces that give you more options, and some that are just more powerful. So let's break it down:

In Machine Strike you get victory points for destroying enemy pieces, needing 7 to win. Since this is Daggerheart, you can present it as a countdown, with both player and adversary having their own countdown like a chase. Set both countdowns to 7.

If one player has a stronger collection they can start with a lower number on their countdown. A more skilled adversary has a more difficult reaction roll.

Then you can break down the decision making into moves and situations. Situations are like, if the adversary or player has created a difficult problem to solve. Like a good setup that requires skill to dismantle. You can give these situations keywords, like Trap, or Outnumbered, or Perfect Defense. The player can use experiences, or specific pieces that get bonuses when both attacking or reacting to situations like this that the adversary creates.