It should be identical to a VPS once you have SSH access.
You need to connect with SSH, move to your flarum install directory with cd and run the composer require vendor/package command there.
It's not even worth a video at that point. The biggest challenge is to know the path to your install once you are logged in. And it will be different with each shared hosting.
Quick Cheatsheet of linux commands to move there:
ls: list the content of current folder (with a shared hosting you are probably already in your htdocs or something like thiscd [dirname]: enter in a directory. Replace [dirname] with the actual name
Maybe you don't know how to login with SSH ? On Linux you should use the ssh command, on Windows you can use Putty. Just Google it if you don't know ?
