Step 1: Collect materials
1.) Speakers - the 6 1/2 variety
2.) Bolts - 8 - 32 fits well
3.) Nuts - make sure to get nylon lock nuts
4.) Female spade disconnects
if you have the speakers it will only cost $7.50 but if not it will be a bit more expensive.
Also, now is a good time to remove those covers for the rear speakers.
Step 2: Cut Holes
Step one is flipping the cover over and then placing the speaker cone down on top. Ensuring that the terminals are in the correct direction dry fit three bolts into three mounting holes, it may take a couple different positions and a bit of wiggling (or drilling if you want) and the bolts should fit through the squares in the cover. The bolts will now be disturbing the fabric on the other side so flip up the cover forty-five degrees off the table and using scissors cut a slit that the bolt can be pressed through.
Step 3: Bolt it
You will need a screw driver and a wrench (or socket) to tighten the nylon lock nuts, but it is worth the effort because these guys will not shake loose.