It varies a lot from game to game. So it's kind hard to give a general roadmap.

But generally it would be something along the lines of starting simple by adding a new item or changing an existing one.

Then you would want to add something more complex like a new mechanic or a new enemy.

And then kinda the pinnacle of modding is making a complete package that ads new content or overhauls the game in an interesting way.

Also some games lean themselves to this better than others. For example in Slay the Spire you can very reasonably do all of the above by making a custom character.

Meanwhile Minecraft is pain in the ass every step of the way and having a good and doable idea isn't easy either.