Once you have created the outline and theme of a game, the next step is to write a set of rules and then playtest and refine. Playtest, refine, repeat.
Your game might sound good in your head or great on paper but it might be terrible once you start playing it. You are going to find rules that aren't clear or ambiguous. You are going to find mechanisms that feel forced or ruin the flow of the game. These intricacies can only be discovered once you start playtesting!
I suggest reading the rulebooks for games that are similar to yours. You can borrow lingo and also reword rules that suit your game. The ultimate test is giving your rulebook to a friend and ask them to teach someone else your game. You will very quickly see where things are unclear and what you understand easily is not interpreted by others in the same way.
Learn from other peoples mistakes and successes.
Throughout my research there are a few things to consider. People don't like games where you are eliminated early and have nothing to do after. This is a major design flaw in games like Exploding Kittens(2015) or Risk(1957).
People don't like games that are too aggressive. People play games for fun and game mechanics like"take that" can be fun when you are dishing out the pain but brutal when things are coming back your way(especially if people gang up on you).
One of our goals creating Big Breakfast was first and foremost to make a fun game, but also to keep players engaged, include several top game mechanisms, create high replayability.
I was going to make it a savage "take that" brawl but then pivoted towards a more amiable version focusing on strategy and hand management.
Overall we are trying to maintain a compelling game, maintain flow, good reward systems, avoid or mitigate negative features, maintain simplicity but also a level of complexity that keeps the game interesting. Nobody wants a game that's too easy or too boring to play again.
*Google any game and rules - there is most likely a website that has the rules for that game. Did I just explain how to Google something...