I have some pretty good experience with this seeing as this is how I made money online. Granted its been a few years, but hopefully some of this can help you.
Preparation
First off pick a engine you want to use. Unreal engine uses C++, and Unity engine uses C#. Both languages are pretty similar, so if you're really good with C++, picking up C# would not be hard at all if you end up preferring Unity over Unreal. You dont have to use those engines, its just what I personally prefer due to the tools they offer. Unity is my favorite choice, because i'm more familiar with the UI.
Next flesh out your idea for the game, make notes of key features, and what the game is actually about. Make a list of what needs to be worked on in order of relevance. It's easy to stretch yourself too thin at this stage, because at first you're probably going to have a ton of features, and want to make the most complete game you can right off the bat. Try and focus on the features that just get the game into a bare bones playable state first.
If you're new to using the engine of your choice then you will need to invest a few days watching tutorials learning the UI, and how the engine works before moving on to the next stage.
Creation
Start coding, and prototyping your barebones game in whatever engine you choose. Don't focus too much on art/sound at this stage. If your game uses a image of a gold coin, dont waste your time making it in gimp, or photoshop, just grab a placeholder from google image search. You can always go back, and replace all the art with original work. The main goal here is to just get a barebones version of your game working.
Refine
Now you're happy with your prototype, its time to clean everything up ( add your own art, sound, new game modes, features etc..) At this stage I normally spend most of my time with art, and UI redesign. After that, start posting threads in reddit, or forums of whatever game engine you picked, asking people what they think, how can you make it better. Take the suggestions, and act on them. While doing this you may end up trending for a little while on subreddits related to the engine you are using.
Pre Release
Make an account with Google Dev Console, it costs $25, and will allow you to publish your game with Google Play. Make a account with Admob, Admob is your monetization tool, or alternatively you can charge for your game. Integrate ads if you choose, and compile your release build, and sign the apk. This will generate a unique key for your apk. Do not lose this key, as you will need it to make updates to your apk in the future. Publish when ready.
Release
Now that the game is on the play store its time to market the game. Post about it on forums, reddit, social media, anywhere you can think, but try not to be spammy about it. Hit up some youtubers and ask if they will showcase it, or talk about it etc. Check your analytics every so often to see where you need to improve, and when you're comfortable with it just start a new project, or keep working on improving.
Side Tips
1. When I first started doing this, I lived in a heavy populated city, so after the game was done, and released I would hit up any phone shop I could, and download the app on a few of the display phones they had. If a person picked up the phone and wanted to test it out, my game was right there already on the device. Potentially thousands of people had eyes on my app per day, so make sure whatever name/icon you choose, is memorable. If a person testing out a phone decides to buy it, they might also re-download your game.
2. If you ever plan on releasing your game to apple users, you need to have an $$$ apple computer to compile the game, or..... setup a virtual OS on a partitioned hard drive. Not saying you should do this, but you can.
3. This somewhat goes into what I was saying in the creation section, but dont over develop. You would be surprised how little people care about graphics and such, as long as the game is fun and addicting, it can look like total hoseshit, and you will still have people playing it. Focus on that stuff last. You will burn yourself out if you try, and make every little thing perfect.
Well, I think that's all I have to offer, but if you have any questions I will try and be of some help.
