1) Open System Preferences.

2) Click Time Machine.

3) Click Select Backup Disk.

4) Choose the disk to which you wish to back up your Mac. If you wish to encrypt your backup, check the box next to "Encrypt backups". Click Use Disk. If you chose not to encrypt your backups, skip to Step 6.

5) Create a password and a password hint to use for your backup. Then, click Encrypt Disk.

CAUTION: Do not forget this password or you will not be able to restore your file(s) in the event of a data loss.

Optional:

6) Check the box next to "Show Time Machine in menu bar".

Outcome

You have created a Time Machine backup.

NOTE: The first time backing up may take a while, but subsequent backups will be much shorter.