Before I became a solo indie developer and made games like Chessplosion and Ducky's Delivery Service, I spent over a decade working for other game studios. I was mostly a gameplay programmer at AAA studios (~150 people) and "big indie" studios (~15 people), and I've done some engine/tools programming and some game design consultant work too.
I was going to write a big post on the game production process but I realized that most of it is pretty obvious. You come up with an idea for a game, then you make sure that the idea is good by prototyping its mechanics and planning everything out (also known as pre-production), then you make the rest of the game (also known as production).
It's just like any other type of art. You think of an idea for a drawing, then you sketch it, then you render the final picture. You think of an idea for a book, then you write an outline and a rough draft, then you write the final novel.
The only part that isn't obvious, judging from how I've seen this exact mistake being made over and over again on all sorts of games, is that you shouldn't move on from pre-production to production until you are 100% confident that the game is already good enough. I really cannot overemphasize how important this is. Ignoring this is by far the biggest and most common mistake that I see game developers make, and it is often game-ruiningly bad.
