What are the requirements of the plugin?
You need at least WordPress 3.9 running on your self-hosted WordPress installation AND PHP in a version greater than 5.3. If you don’t know how to update your PHP version, check this quick blog post for a start: How to update to the latest PHP version for WordPress.
How can I use the plugin?
Once activated, you can find a new “Line Break & Paragraph Spacing” button on your editor: 
In the editor a new image-like field appears. Just drag & drop the size and that’s it!
What is the shortcode that I can use?
The easiest way is to use the button in WordPress’ built in visual editor. But you can also use the shortcode as follows:
[wpb_linebreak height="120"]
Where the height is the space-height in pixels.