To decorate your assistant pages, GtkAssistant provides similar functions to GnomeDruid, so you have to transform code like this:

gnome_druid_page_edge_set_title (GNOME_DRUID_PAGE_EDGE (page), "Welcome");
gnome_druid_page_edge_set_logo (GNOME_DRUID_PAGE_EDGE (page), logo_pixbuf);
gnome_druid_page_edge_set_watermark (GNOME_DRUID_PAGE_EDGE (page), watermark_pixbuf);
    

Into this:

gtk_assistant_set_page_title (GTK_ASSISTANT (assistant), page_widget, "Welcome");
gtk_assistant_set_page_header_image (GTK_ASSISTANT (assistant), page_widget, logo_pixbuf);
gtk_assistant_set_page_side_image (GTK_ASSISTANT (assistant), page_widget, watermark_pixbuf);
    

Where page_widget is the widget used as a page.