If you have lots of time, then you can immerse yourself in the subject. There's tons of reading available, e.g. https://gamasutra.com/category/design/ for starters, and while there are designers who only want to design and have others implement their design, I think you can learn the most and have the most control over your projects if you check out some game creation tools and see what you can come up with (start small, baby steps!)

Me:
I was a freelance graphic designer. I bought a drawing tablet but I never use it until COVID19 hit my country. I was struggling financially up to the point I almost thought of selling my drawing tablet, but then I say to myself, if anything, why not give it a try first, I never drawn on a tablet. I decided to draw stuffs, it was hard. I did it though. My minds thought of making game, since I assumed people mostly will spend time at home, playing games? I look on multiple platform, people sell even the tiniest project they made, it still is profitable. Initially, I wanted to make 3D games with Unreal/Unity. I have start to learned the basics, yet my graphic card died. The easiest game and the best software to run on my low spec PC was Ren'py. I learned python a bit. Making researches and I start drawing more using my Huion420. Today, here I am, releasing my very first game, a visual novel called Memories of East Coast.

For you:
If advice is what you're looking for, I would say, research is a must. Depends on what you planning/ wanting/ dreaming to do, go for that first, from how-to-do, what software, learn the programming language, learn how to design etc. It will be easier, otherwise you might be in dilemma on what you're gonna do, learning too many programming languages at a time and spend most of the time researching but did nothing.

Many had suggested me, for starters, you might wanna go with Unity uses C# which is easier than C++ in Unreal Engine.
You might also want to consider JavaScript, Python or any other simpler programming language.

Method:
- Be imaginative. Collect ideas.
- Find out what you are interested, willing and going to do.
- Depends. Let's say, mobile game. Research on which software offer the best and fits you for building your game. For example, you don't need Unreal Engine if you're going to develop a visual novel. You may use it, but it might be unnecessary to learn C++ for the sake of making visual novel which can be done in Python.
- The same thing from the third point applies on design. If you're going to create 2D games, you don't need a Blender or 3DMaya.
- When you have done all that. Unless you are good an improvising, I suggest do a lil bit homework, including drafting, sketches etc before you start developing.
- Next, research on platform to publish your project.
- Find your audience.
- Marketing.