There’s lots of tutorials out there for cutting spiral steel boning, but many of them are rather vague when it comes to how to apply the caps. Most of them say to just press down on the tips with a pair of pliers. The problem is that when you press down, the sides of the tips flare out. When you press the sides, the top flares up. Neither result is very secure. Some people seem convinced that manufacturers use a special tool for applying the caps, and for that reason they completely avoided cutting the boning themselves and would instead buy it precut to size. Not true! You can apply them like the factory very easily with everyday tools. What you’ll need are a pair of ordinary pliers with a flat end (like these) and a pair of needlenose pliers.
After you cut your boning push on the cap, making sure the top is snug against the wire. Then take your flat pliers and hold the end of the wire so you are squeezing it from the sides. Make sure the tip of the wire sticks out slightly beyond the edge of the pliers so you’ll be able to reach it with the needlenose pliers. You’re using the flat pliers as a vice grip, but you want to make sure you only apply just enough pressure to hold the boning in place. If you squeeze too tightly you’ll probably flop the boning over. The caps are very thin and flexible so not much pressure is needed at all. I’m right handed so I held the pliers with my left hand.
Now take your needlenose pliers and pinch the top to crimp it. Keep pushing out the tip of the boning and crimping the top so you get the entire cap pushed down on the wire. (Ordinarily I would hold the pliers with my left hand and crimp with my right, but I needed to use my left hand for taking the photo.)
Your boning cap will now be securely in place. In fact, the boning cap I applied is probably more flush to the wire than the factory-applied one!