Firstly trim all of the plastic parts so they fit snugly onto the outside of the Helmet.
When that is done, mark the outlines of the parts onto the helmet itself for reference.
If installing an IR Sight into a steel helmet, you may need to cut out a large slot for the IR Sight housing to sit in. This is hidden away under the Strike Plate. This is what happened on the Screen used Helmets for their IR Sights.