Sorry do not have time to study VB but know some basics, here is a simple example
open up your custom window code behind file (MyWindow.xaml.vb)
find where it is like Public Partial Class MyWindow Inherits System.Windows.Window
and change it to be like Public Partial Class MyWindow Inherits
and in the file (MyWindow.xaml) you will need to change the start and end tags from
<Window & </Window to
<Fluent:RibbonWindow & </Fluent:RibbonWindow
you may need to add "xmlns:Fluent="clr-namespace:Fluent"
or similar into the header.