Install this plugin:
Sticky Menu (or Anything!) on Scroll
By Mark Senff
Then, follow his instructions here:
Below, I describe what I did.
Open your page and inspect source. Look the the div of the navigation bar.
In my case it is as shown below:
Copy the class and paste it in the settings for the plugin:
Since it is a class, you need to put a . in front. If it is an id, put a # prefix.
Test
