First, make a small tool yourself: Use a piece of stiff wire. Bend one end into a small hook shape, not too large. Then assemble the hair clip, and use the small hook to push the spring inward until it's in place. Slowly pull out the hook while simultaneously pushing the pin of the hair clip. Once the pin passes through the four clasps, the next step is to fix the spring in the hair clip's slot.
The hair clip has a torsion spring in the middle, and installing this torsion spring is the main difficulty. Before installing the torsion spring on the hair clip, use needle-nose pliers to flatten the two ends of the torsion spring. Don't let go, carefully and slowly insert them into the two clip pieces, and then thread them through the central metal shaft.
Insert the hair clip spring into the nut. Pay attention to the direction of insertion in this step; a slight tilt will make it easier. Insert the spring with the nut into the clip housing and then insert the tool.