At the end of the ignition box wire there is the spark plug boot that is already attached to the wire. Inside this boot is a small spring. Simply push the boot onto the end of the spark plug while slightly twisting the boot side-to-side a little. It will take a bit of pressure but it will push onto the plug and you will feel it when it does.
The outer edge of the ignition boot/cap needs to slide over the metal shell hex of the plug to be considered attached. Again, it does take some effort to "push" the cap onto the plug.

They give you an extra spring with the engine.