Thanks for joining the forum. Its easier that we do questions like this on here, so others can benefit from it in the future.
Following your original email, I documented this today for you.
I am assuming you have the DC04 in front of you without the wand cap and spring in place right now.
First, locate the spring in the holes on each side:
Observe the direction the kink in the spring sits. Make sure you get it the right way round.
Then, look at your wand cap. There is a ridge running down the centre on the inside. That ridge has a little nick in it. See the photo below:
The end cap fits onto its round locating lugs with your fingers. You simply expand it a little to locate it on the lugs as you fit it. Be sure not to disturb the spring from its little holes as you do so.
Having fitted it, all you need to do now is locate the spring in the little nick. You do this by placing a screwdriver carefully from above like so:
Push the spring down the ridge until it locates into the little nick. It will snap in. Check the spring is level and not at a tilt at either side. If it is, manipulate it straight using your screwdriver, taking care it stays in the slot.
That's it! Your spring is fitted.
Let me know how you get on.