It will be a lot clearer to you when you buy what you want.
The links you posted are active subs, this means the amp is all built in to the wooden box along with the sub.
your diagram shows a setup with a separate amp, but you in your diagram are earthing the sub whereas with a separate amp you would instead earth the amp.
In an active setup ( like the links you posted) there are no connections to be made between the sub and the amp.
between the sub/amp and the car you will have to supply power from the car battery and a good earth.
between the sub/amp and the stereo itself there will be a blue cable ( which tells the amp to switch on and off with the hu) and some RCAS' these are dual cables and normally have red and white ends, these cables carry the sound from your stereo to your sub/amp in the rear of the car.
The only real difference from the above when you use a separate amp is that you need to make two further connections between the separate amp and the sub box. This is just like wiring up your home stereo, its easy. If you use a separate amp the connections for power and earth would be on the amp, instead of the side of the sub box.
The most difficult part is finding a good earth, any old earth will normally not be too good. The next most difficult part will be routing the power cable to the battery through the bulkhead of the car. The next most difficult bit is normally getting the wires through your dash from your HU ready to be routed down to the rear of your car. The rest is normally fairly simple.
