In short, if you aren't technically minded, don't bother. I've just spent the best part of 4-hours changing the fascia on mine. You have to completely dismantle the phone which means disconnecting ribbons and removing around 20 screws from various components. 

The cheap chinese fascias in ebay don't include a few of the key components required for proper operation of the phone. If you are going to do it you'll need to remove the lenses from your old phone.

The most important part missing from these cheap fascias is the magnet which tells the phone the slide is up or down. It's in the screen section of the fascia, in the middle at the bottom, probably covered by a bit of tape.

After a couple of heart sinking moments I have finally got mine back together and working fully. It really is a big job and well worth paying someone to do it. It's also worth paying the extra and getting a genuine samsung fascia as the one I bought, the colour is already starting to come off and doesn't feel as good as the OEM version.

Really, don't tackle this unless you are confident with electronics and have a good couple of hours in which to do it.