Please refer to documentation on the Darktable website

https://www.darktable.org/install/#ubuntu

And the .debian.tar.gz files available from the Darktable PPAs:

https://launchpad.net/~pmjdebruijn/+archive/darktable-release/+packages
https://launchpad.net/~pmjdebruijn/+archive/darktable-unstable/+packages
