Dafterz the command is actually like the following, but before I explain you can get help in composer for the command using help:
composer help create-project
Now to install flarum:
composer create-project <vendor>/<package>:<version?> -s <stability> <directory>
So:
composer create-project flarum/flarum -s beta .
The . (dot) indicates the target directory, in this case the current one, you can use a full path or a relative path.
In case you want to install into subdirectory forum:
composer create-project flarum/flarum -s beta forum
Please be warned, the target directory must be empty.
