Source: https://github.com/backdrop-contrib/drush
Create Drush custom commands folder
mkdir -p ~/.drush/commands
Get Backdrop Drush commands
cd ~/.drush/commands
git clone https://github.com/backdrop-contrib/drush.git backdrop
Use latest stable release
cd backdrop
git checkout $(git tag | tail -1)
Refresh Drush commands cache
drush cc drush
cd ~/.drush/commands/backdrop
git fetch --tags
git checkout $(git tag | tail -1)