@kyzoe-be I've never used the LAMP app myself (because part of the whole reason for using Cloudron is to not have to bother), but I think the idea is just to install Drupal like you would on any other LAMP server as that is basically what the LAMP app is.
So I'd guess you just follow standard instructions for installing Drupal like e.g. https://www.drupal.org/docs/installing-drupal