Make sure the following are installed:

ffmpeg, dvdauthor, genisoimage, growisofs, dvd+rw-tools

enter the following:

$ export VIDEO_FORMAT=PAL      # This is used later when the DVD is being authored.

now need to convert the video file into mpg format:

$ ffmpeg -i yourfile.mkv -target pal-dvd -aspect 16:9 yournewfile.mpg

Now we can use dvdauthor

$ mkdir DVD
$ dvdauthor -o DVD -t yournewfile.mpg     # -o defines the output folder, -t defines the title file. 
                                          # Repeat this for as many files as you want on the dvd.
$ dvdauthor -i DVD -T                     # This defines the TOC.

We are now ready to convert the DVD to an iso to burn

genisoimage -dvd-video -o yourimage.iso

Once this is ready to burn can use k3b or on the command line use:

$ growisofs -dvd-compat -Z /dev/sr0=yourimage.iso

Ref: https://www.linux.com/blog/how-convert-video-file-iso-burn-dvd