How do I install a Seagate backup external device

Reformat the drive using your Disk Utility.app --> choose one partition, GUID extended journeled.

Download and use the trial version of Carbon Copy Cloner software to back up your internal drive to the external drive.

https://bombich.com/

Easy.

I suggest that you consider Time Machine to backup your MBP. It is easy, straightforward and runs in the background:

https://support.apple.com/en-us/HT201250

http://pondini.org/TM/5.html

Ciao.