Assuming you want to add some fancy graphic at the top of the banner. Generate one using TAAG.

1. Edit the file /etc/update-motd.d/00-header
2. Using printf add the banner title. Remember to add newlines \n) at the end of each printf (good old C style), e.g.

   printf "\n\n ██╗   ███╗ █████╗ ██╗  ████████╗ █████╗\n"
   printf "████╗ ████║██╔══██╗██║  ╚══██╔══╝██╔══██╗\n"
   printf "██╔████╔██║███████║██║     ██║   ███████║\n"
   printf "██║╚██╔╝██║██╔══██║██║     ██║   ██╔══██║\n"
   printf "██║ ╚═╝ ██║██║  ██║███████╗██║   ██║  ██║\n"
   printf "╚═╝     ╚═╝╚═╝  ╚═╝╚══════╝╚═╝   ╚═╝  ╚═╝\n"

3. To assemble and update the MOTD, run sudo run-parts /etc/update-motd.d/. This will concatenate all output of the scripts in that directory.