You can install Stacer in a number of ways. The simplest process is to use the Ubuntu Software Center.
Open Ubuntu Software Center and do a search on Stacer:
Click on Stacer and follow the prompts to install the Stacer.
To install darktable via terminal. Open terminal via menu or via keys CLT+ALT+T.
sudo apt install stacer
You can also use Stacer as a no-install App-Image which you can download from Here.