Using the command line
As root, or using sudo, open your sources.list file (Nano is the recommended editor for new users):
# apt edit-sources
Append the following line to the bottom of the file:
deb http://deb.debian.org/debian buster-backports main contrib non-free
If you are a free software enthusiast, you might want to remove the contrib and non-free sections. (See Debian package management for details.)
Now that you have added the repository, update APT's cache to include the backports in the list of available packages:
# apt update
Done.