there's a guy named hal9000 that used to post here, but i haven't seen him around for a while.
He did a thread that covered this process in detail, but it was for a small combo, not a 4x12.
The annoying part about this is that there are literally hundreds, if not close to a thousand, big staples that you have to remove by hand. Each staple is tapped in with a high pressure gun, so they are not very easy to remove. You have to do it with a flat-head screwdriver or knife.
After that, you place the new cloth on, stretch it very very tight, and get to work stapling. Then you mount it back on and put the piping.