Give your nav a name. This will be represented below with "slug" and for the purposes of illustration, we will call our example "microsite".

Next, add the following tag to the pages that you want in the nav and replace "slug" with the name of your nav menu:

slug_nav

i.e. microsite_nav

Then, add the following tag to all of the pages that you want the nav to show up on (this will replace your site's top nav on these specific pages):

nav tag: slug_nav

i.e. nav tag: microsite_nav