@luceos. Here I am, composing this long reply, and when I am almost finished, you beat me to it! ?
@alishan creating a uniform tutorial for using composer is not as easy as it sounds. If you manage your own VPN/server with SSH access, it is fairly straight forward, but if you use shared hosting, it really depends on your hosting setup.
More and more hosts allows ssh, and some of those again allows composer. Some of those again needs you to setup composer yourself, while some come ready to use. If it is ready to use , all you need to do is to follow their guide on how to log into ssh and then use the flarum install instructions.
If your host does not include SSH and or composer, I might have written a web installer that might work for you: https://discuss.flarum.org/d/5731-pockethold-a-web-based-installer