Its easy to do it yourself. Just buy an amp install package from supercheap or a similar shop, however if you have high quality gear, get a high quality kit (an install package/kit will come with a set of RCA leads, a remote wire, a positive power wire and a ground wire).
Then basicly the RCA leads and remote wire run to the cars head unit, the positive wire runs directly to the battery (make sure you have a fuse installed near the battery) and the negative wire attaches to the body of your car (generally you connect it to a screw in the boot, assuming it is earthed to the body). The remote wire allows the amp to turn on when the stereo is turned on and the RCA wires transfer the audio signals to the amp.
Depending on the brand of the amplifier and the RMS wattage output, it generally isn't worth running two subs from two of the back channels of an amp (the 3rd and 4th channel have the low pass filter and are intended to power a sub) - Its normally better to bridge one subwoofer accross two channels, otherwise they might nor have enough power.
However in your case, if you're really keen I'd get another amp the same as the one you are getting in the package and then I'd run all your normal car speakers from the first two channels on each amplifier (channels 1 and 2). Then bridge the subs accross the two channels (channels 3 and 4, bridge accross them) of each amplifier. So you would have a total of 8 channels and the speakers would use 4 channels and the subs would use 4 channels - With this install you would have to have 2 sets of power leads running from the battery and the cost would obviously be more. And depending on your car, as Naz said you may have to purchase a capacitor (its basically like a second battery that temporarily stores power) or upgrade the alternator in your car (this would be pretty rare, but depends on how far you go). The reason behind this is that subs play the music below a certain frequency and when they come into action (when the bass signals play) they suck alot of power and you can have times when the car can't give enough power to give the amps and you get bad distortion.
The amp and the wire kits will come with instructions and diagrams to install. Playing with car audio is alot of fun and you can always be upgrading your gear, but remember things can go wrong, but they are generally easy to fix.
Oh and when I used the term "bridge" this means run one sub from two amp channels, basicly combines their power and allows them to go louder
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