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"

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

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

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

Check this blog post. UML sequence diagram in this page is generated with this plugin. Blog sources are available in Github.