The first thing to note is that, like @anon19508339 already found out, Flarum is standalone forum software. Flarum is not something you can install “in WordPress” because there is not relation with WordPress. If you want a forum integrated in WordPress, BuddyPress used to be the way to go, but I don’t know whether it’s still popular these days.
Also, I wouldn’t call Discourse hard to install, but simply impossible to install on any web hosting service. Flarum, on the other hand, is what I would consider hard to install.
That is because Flarum requires Composer to install, and running Composer on free hosting is not possible.
So to install Flarum, I would install Composer on my own computer first, then use Composer to create a new Flarum site locally. Then, you can upload the contents of the generated Flarum site to your hosting account.