ssd to back-up my ma

Plug it into your USB port, startup Time Machine and use it.

Back up your Mac with Time Machine - Apple Support

"If the disk you selected isn't formatted as required by Time Machine,

you're prompted to erase the disk first. Click Erase to proceed. This

erases all information on the backup disk. "

FireDave7 wrote:

How to i use an external ssd to back up my mac via time machine?

All new drives need to be formatted.

Erase and format a storage device in Disk Utility.app on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/19.0/mac/10.15

Backup disks you can use with Time Machine

Back up your Mac with Time Machine: https://support.apple.com/en-us/HT201250