A few useful things to help you get started.
Some base HTML for the propositional navigation is included in the views/includes. You should include it as a partial in your views.
As explained in the govuk_template readme, you also need to include the headerClass
tag, set to the following in your template to add that class to the #global-header
element:
{{$headerClass}}with-proposition{{/headerClass}}
An example of this can be seen in the layout.html template.
You can add the styling for the Alpha or Beta phases by including the phase tag in your propositional navigation and adding the banner to your template.
The phase tag should be added to your service name in the propositional navigation. See the propositional_navigation_alpha.html partial for an example.