Hi Tuan
It's pretty straightforward, the 3mm ply was a perfect size for the speaker mounts just be careful when cutting the holes as you do get quite close to the edge! If I was to do this part again I would probably use 3mm mdf for a little extra stability
I used alcantara to cover the mounts (as I had some left over from previous mods)
I ran the wires under the centre console and behind the rear carpet
You have to check the link Sparker posted on wiring multiple speakers as it explains everything very well.
http://www.termpro.com/articles/spkrz.html
If you decide to go ahead and have any questions send me a PM and I will help if I can.