Backing up to Portable Drive for Mac

I’m currently running an external hard drive for Mac. I’m trying to back up my system. I’m running Catalina 10.15.7. My WD portable for Mac is 96% free so there’s enough space. My last backup was August so I’m currently stuck in preparing backup

Hi,

You could refer to the following link: https://support-en.wd.com/app/answers/detail/a_id/11433