this is my least favourite repair...loads of patience needed, lots of frustration. Take a look inside the switch that you use to turn vibrate/mute on and off. You'll see the tiniest of grooves and it is this that you've got to put a tiny piece of plastic switch into from the flex.remember that the red dot is toward the top of the phone on the outer switch..
easiest way of doing it is with some sticky tape or blue tak to hold the vibrate switch from the outside of the phone so that you've got 2 hands free to fiddle about trying to get the tiny plastic toggle on the end of the flex into the groove that's on the vibrate outer switch. You'll need to have the switches both in the down position. once that is in you can put the volume switch in. It's pretty straightforward, no tricks here. The flex folds on itself as it passes around the top of the battery...just lay the flex from power to volume and you'll see that if you were to stretch the flex out it would reach to the bottom of the phone (obviously a silly idea) don't fold it, just double over so that volume controls go in. the upper end of the metal holding the volume button in sits over the top of the metal clasp holding the mute assembly in. lastly the power button... put the metal ring dangling down, it sits on a very small "lip" on the rear case. put screws in and tighten enough to hold power button in...try the button and make sure you have "bounce" if too much tighten a little more, no bounce then loosen. I think there is 1 more screw to secure flex near volume/mute to bottom of case. good luck and happy swearing!
the metal pieces should secure to the flex ends with double sided tape which should be provided on your purchased part. it should be self explanatory. the "nipple" portion obviously sits on the outside of each switch to make contact with the outer button. If it's too annoying send it back and order one with the metal plates attached!