For the COM wire, find another COM wire on the original connecter, cut it, strip the ends of the wire, solder them together and then insulate it (heat shrink tubing or insulating tape). You'll end up with something like this:

Do the same to the other connecters. Then double, triple check what you have done. Getting someone else's opinion also helps here. Even if they don't have the first clue about electronics, their questions help you spot mistakes. If it's wrong, you'll cause a very nasty short circuit somewhere and kill something somewhere (hopefully only the fuse in the PSU). You have been warned.

Also note there is a 5v feed to the ATX plug which is different to the others - it's denoted as 5VSB. This feed is always on; it powers the motherboard even when the computer is turned "off".

Do not use this on the BTX mini connecter. I cannot stress this bit enough! Ensure that it looks exactly as in the diagram shown - if the square and house shaped plugs are orientated the wrong way compared to how you wired it, you will end up putting it in the wrong way round and giving the wrong voltage to the wrong things.

Last note: The new COM wire on the BTX connecter used to be a PS_ON wire and is often quite thin. If you can, you might want to try and directly solder the COM wire directly to where the PS_ON wire is connected to the plug contact so any current flowing doesn't end up flowing though a wimpy little wire. This, however, is almost certainly not necessary, as all the COM pins are connected to each other on the motherboard anyway.

Good luck!

Roy Dennis