On Debian based GNU/Linux distributions you can install it via the package manager:
sudo apt install plantuml
Assuming that you use use-package to manage your Emacs config, you can add this code to your emacs config file:
(use-package plantuml-mode)
(org-babel-do-load-languages 'org-babel-load-languages '((plantuml . t)))
(setq org-plantuml-jar-path (expand-file-name "/usr/share/plantuml/plantuml.jar"))
(setq plantuml-default-exec-mode 'executable)
#+BEGIN_SRC plantuml :file dead_parrot_sketch.png
title Dead Parrot sketch
Mr.Praline -> Owner: 'Ello, I wish to register a complaint.
Mr.Praline -> Owner: 'Ello, Miss?
Owner --> Mr.Praline: What do you mean "miss"?
Mr.Praline -> Owner: I'm sorry, I have a cold
#+END_SRC
Create a new file that ends with .plantuml(or run M-x plantuml-mode if the file doesn't end with .plantuml) to enable plantuml-mode and have syntax-highlighting, auto-indentation etc.
Write/update the source code and run C-c C-c to generate a preview of the diagram.