Some instructions here because the porting guide in the Gtk+ docs is quite short at the moment:

Convert the files

gtk_builder_convert <old_glade_file> <new_gtkbuilder_file>

Code changes

• Remove #include <glade/glade.h>
• GladeXML* => GtkBuilder*
• glade_xml_new (FILE, “first_widget”, NULL) becomes
  
GError* error = NULL;
GtkBuilder* builder = gtk_builder_new();
if (!gtk_builder_add_from_file (builder, FILE, &error)
{
g_warning ("Couldn't load builder file: %s", error->message);
g_error_free(error);
}

• glade_xml_get_widget (gxml, “widget_name”) becomes GTK_WIDGET (gtk_builder_get_object (builder, “widget_name”))
• glade_get_widget_name (widget) can be replaced by gtk_widget_get_name(widget)
• glade_xml_get_widget_prefix (gxml, “prefix”) can be emulated by gtk_builder_get_objects(builder) together with manual filtering. It returns a GSList* instead of a GList* though.

That’s it basically. Don’t forget to adjust your Makefile.am to install the correct files and update POTFILES.in!