Install it first:

$ gem install jekyll-plantuml

With Jekyll 2, simply add the gem to your _config.yml gems list:

gems: ['jekyll-plantuml', ... your other plugins]

Or for previous versions, create a plugin file within your Jekyll project's _plugins directory:

# _plugins/plantuml-plugin.rb
require "jekyll-plantuml"

Highly recommend to use Bundler. If you're using it, add this line to your Gemfile:

gem "jekyll-plantuml"

The plugin is compatible with Jekyll 3.9.3 and Jekyll 4.3.2. You can find our integration tests, which prove the compatibility, here.

Then, make sure PlantUML is installed on your build machine, and can be executed with a simple plantuml command.

On Ubuntu, just apt-get install -y plantuml should work. However, if it doesn't, you can create a /usr/bin/plantuml with the following content:

#!/bin/bash
java -jar /home/user/Downloads/plantuml.jar "$1" "$2"

Remember to change the path to the plantuml.jar file.

Then, set the executable permission of the file:

$ chmod +x /usr/bin/plantuml

Now, it's time to create a diagram, in your Jekyll blog page:

{% plantuml %}
[First] - [Second]
{% endplantuml %}

Now, check this blog post: the UML sequence diagram in it is auto-generated using exactly this plugin. The sources of the blog are available in GitHub.

Read these guidelines. Make sure your build is green before you contribute your pull request. You will need to have Ruby 2.3+ and Bundler installed. Then:

$ bundle update
$ bundle exec rake

If it's clean and you don't see any error messages, submit your pull request.