philwyett@ks-kenobi:$ dget -x http://deb.debian.org/debian/pool/main/f/ffmpegfs/ffmpegfs_1.10-2.dsc
dget: retrieving http://deb.debian.org/debian/pool/main/f/ffmpegfs/ffmpegfs_1.10-2.dsc
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  2028  100  2028    0     0  26684      0 --:--:-- --:--:-- --:--:-- 27040
dget: retrieving http://deb.debian.org/debian/pool/main/f/ffmpegfs/ffmpegfs_1.10.orig.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 49.0M  100 49.0M    0     0  4036k      0  0:00:12  0:00:12 --:--:-- 4329k
dget: retrieving http://deb.debian.org/debian/pool/main/f/ffmpegfs/ffmpegfs_1.10-2.debian.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  6192  100  6192    0     0  88457      0 --:--:-- --:--:-- --:--:-- 88457
ffmpegfs_1.10-2.dsc:
      Good signature found
   validating ffmpegfs_1.10.orig.tar.gz
   validating ffmpegfs_1.10-2.debian.tar.xz
All files validated successfully.
dpkg-source: info: extracting ffmpegfs in ffmpegfs-1.10
dpkg-source: info: unpacking ffmpegfs_1.10.orig.tar.gz
dpkg-source: info: unpacking ffmpegfs_1.10-2.debian.tar.xz
dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: applying patch-create-no-user
philwyett@ks-kenobi:$ cd ffmpegfs-1.10/
philwyett@ks-kenobi:$ dch --bpo 
philwyett@ks-kenobi:$ dpkg-source -b .
dpkg-source: info: using source format '3.0 (quilt)'
dpkg-source: info: building ffmpegfs using existing ./ffmpegfs_1.10.orig.tar.gz
dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: building ffmpegfs in ffmpegfs_1.10-2~bpo10+1.debian.tar.xz
dpkg-source: info: building ffmpegfs in ffmpegfs_1.10-2~bpo10+1.dsc
philwyett@ks-kenobi:$

Note:

When you do dch --bpo, it will open the debian/changelog in your preferred editor and should look similar to below.

ffmpegfs (1.10-2~bpo10+1) buster-backports; urgency=medium

  * Rebuild for buster-backports. 
  *

 -- Phil Wyett <philip.wyett@kathenas.org>  Fri, 24 Apr 2020 18:25:46 +0100

Remove the unnecessary * and empty line, then save the file and exit the editor.

You can always reopen the debian/changelog to make sure all your changes are correct before continuing the session above.