How to back up your Mac with Seagate One Touch

This will destroy all data on the disk. Open Disk Utility select the Seagate drive on the left sidebar and click the Erase button. Recommend that you use APFS for the filesystem.

Then eject the disk and reconnect it. You should see a prompt from macOS asking if you wish to use this disk for Time Machine.

An alternative to Time Machine would be CCC - Carbon Copy Cloner which is a wonderful way to backup a Mac. Time Machine works well for most people but I really like the way CCC works. It gives you more control and can be more reliable than Time Machine.