Mark out the insert to allow for a maximum gap between spindles of 100mm. Screw through the top and bottom inserts into the spindle ends. Then using six or seven shorter screws so that they don't poke through the handrail's upper surface, attach the insert to the top and bottom rails. The balustrade panel can now be fitted between your newels using a suitable balustrade bracket.
Please note: For safety reasons, if the decking surface is 600mm or more above ground level, then the height of the handrail top must typically be a minimum of 1.1m above the surface of the decking boards. In addition, the gap between the deck surface and the bottom of the base rail should generally be 100mm maximum.