Try adding in some windows using glade:
Open up a terminal, go to your project directory and type "quickly edit" - That will launch glade (gui designer) and it will automatically load in your project's ui files. In glade you can add windows by going to the Toplevels area on the left side and choosing which type of window you want.
To add widgets to that window, you have to put a container of some type into your window. A container will hold the widget basically. Once you have a container you can add as many widgets as it can hold.
If you would like to add windows in using Python:
I would recommend reading this tutorial.
To make a window with a button, try doing something like this:
from gi.repository import Gtk
box = Gtk.Box(spacing=6)
button = Gtk.Button("Test Button") #create a button
button.connect("clicked", self.on_button_clicked) #connect the handler
box.pack_start(button, True, True, 0)
def on_button_clicked(self, button): #what happens when button is clicked
print "Button was clicked!"
win = ButtonExample()