Macbook Pro
How do I start backup with a Time Machine My Passport?
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Attach your external drive and use
System preferences > Time machine ...
... to turn on Time Machine and specify what drive to store your Backups on. It may ask to initialize the new drive, and that is as expected.
Time Machine may spend all afternoon making your first full backup. You can continue to do your regular work while it does this. The first Full Backup is by far the biggest backup. After that, it will work quietly and automatically in the background, without interrupting your regular work, and only save the incremental changes.
Time Machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.
How to use Time Machine to Backup or Restore your Mac:
Back up your Mac with Time Machine - Apple Support