Cerberus installation is done automatically, but because of specificity of security application, mast to pay maximum attention application to be installed on clear, as much it possible, from malware site.
 So, mast to be taken at least some simple steps for site clearing:
 1. Download your Joomla site and database backup SQL.
 2. Get fresh download of your site version Joomla from https://www.joomla.org/. Unzip.
 3. Scan the copy of your site backup, with your installed antivirus. Focusing primary on images, media and extensions folders, from admin and site areas. Since Joomla core files will be replaced by fresh and clear ones.
 4. Via FTP or terminal remove everything from site root folder. Use server root FTP if available.
 5. Inspect parent to root folder for malware absence:
  a. Download parent to root folder (without, of course, site folder).
  b. Scan, downloaded parent folder, with your installed antivirus.
 6. Via FTP upload fresh unzipped Joomla files from step 2.
 7. Remained to install your extensions you used.
  a. If from step 3. checking folders (images, media and extensions folders) are ok - upload them so. At this point, please, be careful. Often, some intruders so cheeky so they, expose to a common view their scripts without any obfuscation and antivirus simply pass them as legitimate. So, please, examine thoroughly all executable files.
  b. Else inspect carefully from backup one by one extensions file by file. and upload them if ok. This usually a simple and fast task.
  c. Or install your extensions if you are unsure for files integrity.
 8. Inspect language files - usually they always ok.
 At end install the Cerberus plugin, which automatically will take site clear copy and record to database file system data.
 Finally, don't forget to take an complete site backup files and database
 If you follow these simple steps you'll get protect your invaluable assets, effort you invest for your internet presence and gain the peace of mind about your site safety, which today is an invaluable asset so.