Lets assume you wanted some app from buster-backports, usually because its more up to date than the version in buster. The Raspberry Pi foundation doesn't have a backports repo so you have to use the Debian one instead. This is how to add it.
Update: March 2021
The Raspberry Pi OS 64 bit already has the necessary signing keys. I
haven't used the 32 bit one for a while but it may also have the
necessary signing keys. You only need to do point 2 if you get an error telling you you don't have the GPG keys.
As mentioned in the comments the GPG keyservers disappear. The kfwebs.net listed below is no longer available but there are others listed in the comments that can be used instead.
1. Add to /etc/apt/sources.list
This adds the Debian buster-backports repo to our list of repositories to use. To edit the list of repositories:
sudo nano /etc/apt/sources.list
Add the following line to the end
deb http://deb.debian.org/debian buster-backports main
2. Add Debian Buster keys
This adds the signing keys so we can trust the Debian repos.
gpg --keyserver hkp://keys2.kfwebs.net:80 --recv-keys 04EE7237B7D453EC
gpg --keyserver hkp://keys2.kfwebs.net:80 --recv-keys 648ACFD622F3D138
gpg --export 04EE7237B7D453EC | sudo apt-key add -
gpg --export 648ACFD622F3D138 | sudo apt-key add -
3. Tell apt to update
Tell apt to get the latest list of packages.
sudo apt update
4. See if there is an updated package
If you want to see if there is an updated package called xxx use the following command:
sudo apt list xxx -a
5. Get a package from the backports repo
By default apt won't install packages from the backports repo, you need to tell it. If we want to install package xxx from buster-backports use the following command:
sudo apt install -t buster-backports xxx