taking the existing deck out is fairly easy...

You need to undo the gear knob, then dig your hands into the shift boot and then slowly pull out the bottom half of the central dash, they are all clips. Then unclip the wire harnesses. You should then see two screws, undo those.
Then to remove the top half, simply pull gently to remove it. The rest are also just clips. Unclip those harnesses as well.

Hope that helps.

I haven't changed my speakers yet but planning to later...