iTayGNR Pretty much every extension posted here has the composer installation command in the first post. If you don't see one, link them here and we can help you figure out the right command. It's usually vendorname/packagename which generally winds up in the format github_username/flarum-ext-extension-name.
As for actual usage, make sure Composer is installed on your server (following the instructions at getcomposer.org) and you are in the flarum root folder. You can check this by running php flarum info and getting a successful response back.
Simply run the command, e.g. composer require vendor/packagename and let it go. Then refresh your admin/extensions page on your flarum installation and enable your new extension.
