How should I approach (or go about) developing games?

I'm a lone C# programmer and I'm in the middle of developing an early prototype of a game but I feel as if I'm just shooting from the hip. I feel as if even though I've created my own GDD (Game Design Doc) complete with sprint cycles, I'm not going about this whole process professionally.
What I would like to know is how do the professional game developers approach game development?
Like, do they first create some sort of framework of the game first and then expand into additional content within that framework?
Do they create data models, use cases, UML diagrams and such to dictate how the game is produced?

It would be great if I had a mentor in this but in the absence of that the internet will do! :-D

If you've created a game or know of some good resources that would help my questions, please don't hesitate to reply.

Thanks!