g drive

Connect the G Drive directly to your Mac and power things up

On your Mac.....

Click on the Time Machine "clock" icon at the top of the Mac's screen

Click Open Time Machine Preferences

Click Add or Remove Disk

Click on the G Drive to highlight it

Click Use Disk

If Time Machine is operating correctly, it will automatically format the G Drive for you correctly and a backup should begin shortly.

If Time Machine cannot format the drive automatically, then you can do this manually. The article linked below should help with this.

https://osxdaily.com/2012/01/04/format-an-external-hard-drive-or-usb-flash-drive-for-mac-os-x/

G drive will plug directly into your Mac. You must have it formatted HFS+ to work with Time Machine.

However on Catalina I recommend using Carbon Copy Cloner or equivalent and format the drive APFS to create a bootable clone.. that is if this is a desktop drive not laptop type drive which are too slow for APFS format.

Can't think of anything that you should know before plugging it in.

Might you really be asking "how" to set things up?