How do I back up my Mac to My Passport for Mac an external hard drive

• Connect My Passport and follow the instructions (you'll be asked if you want to use use the external drive to make time machine backups)
• If you want to see the TimeMachine icon on the menubar (on the right sight), click on the apple icon on the menu bar and click on SystemPreferences
• Click on the TimeMachine icon (4th row)
• Unlock at the bottom (left side by giving in your Mac password)
• select 'Show TimeMachine on the menubar)
• a first Time Machine backup can take a long time
• don't use the external drive for something else than the Time Machine backups

Just use Time Machine, it's already on your Mac Back up your Mac with Time Machine - Apple Support

or clone the drive using either Carbon Copy Cloner https://bombich.com/ or Super Duper https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

Time Machine is not an external device, it's software from Apple that will back up your data to an external device like the one you already have. See the support document I linked to for details.

Carbon Copy Cloner and SuperDuper are also software solutions that will use your existing external drive regardless of how poor of a choice it is.

"Thanks but I would prefer to use the one I have."

Mike has already told you about Time Machine and link

to the pages on how to set up and use!