1) Find A High Quality Source Code.
Start making your researches and find a good quality source code that you can reskin with. iKodeGames is a very decent marketplace where you can buy mobile app/games templates and reskin them very easily with the instructions provided.
2) Start Reskinning the Source Code
When you have purchased a decent source code, you should start by reskinning it. In order to do that, you can do it yourself by creating new graphics in-game or hire a graphic designer to do the job.
3) Test the mobile app
Before publishing the mobile app to the google play/apple store, start testing the new game with your friends/co-workers. This step is crucial for your mobile app because you will be eventually fixing some final things.
4) Launch the mobile app to the google play/apple store
When Reskinning is done, when Testing is done, you can finally publish your new app to the google play/apple store. If you do not know how to do that, usually when you buy a source code, the owner of it can provide you clear instructions regarding that.
5) Start promoting your mobile app via Google Adwords
When you eventually launch your mobile app, you can start promoting it via Google Adwords. This would be the best way to make your app visible to the world. Limit yourself to a daily budget and watch magic come in!