NOTE:This is an exact duplication of ChattavegasSE's how-to. I did not write this how-to, I only made this post, rehosted the pictures so they would be available again, and stickied the new version.
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I've seen a lot of posts about this, and I've even gotten a couple PM's, so I figured I'd do my civic duty for the year and write another long how-to. First off, I haven't done this yet, but I know how to do it. The only thing I don't know is the exact length of cable needed to go from front to back.
You need:
1) 0ga cable (I think about 18-20 feet is about right, but again, I'll know when I do it)
1a) don't forget you have to ground the new battery in the trunk, and ground the front distribution block, so order cable to do that, too. About 6-8 feet of 0ga oughtta do it.
2) 2 of these distribution blocks Item #: PB-2044
3) 2 battery terminals (Optional-looks a million times better with them) Item #'s: BT-104N and BT-104P $12/ea
4) 1 ANL fuse holder (or circuit breaker) Item #: FH-20 $20 and a 200A ANL fuse $6
4a) a more pimpadelic fuse holder (if you're running amps in the trunk) is the digital ANL fuse block Item #: FB-1024D $40 (not required-just a cool upgrade)
5) Ring terminals - depends on how you want to attach the cable to the battery. Using the battery terminals in #3, you can either plug the cable straight into the terminal or use a ring terminal and screw on the top. If you decide to use the screws on the top, you'll need 2 0ga rings - they come in black, blue, and red. Item #'s: RT1/0516BK RT1/0516BL RT1/0516R $1.75/each. You will also need one ring terminal to use for the ground cable. You also need to ground the front distribution block in the front of the car, so you'll need another ring terminal for that. That's two rings you definitly need, and two you can use if you want.
I have a pair of set-screw ring terminals instead of the crimp style rings on my battery. They look like this:
6) Some kind of mounting bracket to hold the battery, either a battey box or tray. These can be had on ebay or Summit or Jeg's. Maybe even your local rice shop sells 'em, I don't really know where you live. Heck, you might even be able to use the stock battery box and modify it to mount to a flat surface instead of the curved fender. Ghettolicious!
Now that you have all your new toys, get out there and do it! How-to:
1) Take out the old battery.
2) Decide where you want to mount the two distribution blocks. The battery case is a good choice, unless you have an evil idea for CAI or something. Bear in mind the length of the wires that went to the battery. Screw the blocks down into place where you want them.
3) One side (positive/negative) at a time, cut off the OEM terminals that attached to the battery so you have bare wires. Do this one side at a time so you don't get them mixed up, as most are black. See pic:
4) Attach the wires to their respective power block. I would put the positive block on the passenger side and the negative block on the driver side. The battery box can easily be modified to have the wires come in through the back, just drill some holes in it. Don't put any of the OEM wires in one of the 0ga input/outputs of the distribution blocks. There are 4 4ga ports on each block, use those.
5) Repeat steps 3 and 4 with the other side (positive/negative)
6) Now we're gonna ground the negative block in the front. Get your length of 0ga cable and a ring terminal. There are countless ground points in the engine bay. Find one close to the distribution block and use that. Be sure to strip away the paint around it so the ring terminal sits on bare metal and not paint. If you're worried about rust, a quick can of spray paint will set you right. Try to use the nearest ground point so as to minimize the length of the ground wire. The 0ga ground wire will connect to one of the large openings in the ends of the distribution block. Like this, more or less:
Cut off the excess and your done with that step!
7) Okey dokey, now let's run the positive cable to the trunk, k? OK! Connect the 0ga cable to the positive distribution block. Now, there are a lot of different ways to do this. Some people run the wire under the car and snake it through the chassis so as to protect it. While other people run the wire through the firewall and under the carpet; either under the door sill or under the center console. Knu Konceptz' 0ga compressed cable would make it a lot easier to run through the inside of the car.
7a) getting through the firewall:
ATX cars: there's a large rubber tear-drop-shaped grommet where the clutch cable goes in manual cars. Run the wire through there, it's easier than taking candy from a nun.
MTX cars: there's a grommet higher up the firewall just under the wiper cowl. You can use that to let the wire into the car, or just drill a new hole, which would probably be easier.
7b) Pull the wire through the firewall so it's almost tight on the distribution block. You don't want any slack, so you aren't wasting wire. You now have all the positive cable bunched up inside the car, right? Right! OK! Now you have two options, A) run the wire under the driver's side door sills or 2) run the wire under the cente console and carpet straight up the middle. In this how-to, I'm gonna go under the door sills. So now, you do it, too. C'mon, it's fun, right? Just like installing an amp, pull the new cable under the back seat and into the trunk.
8) Now let's figure out where/how to put in the battery. Pick your location. In this how-to, I'm going to put mine in the spare tire well, just behind the spare mounting bracket. Mount the battery in whatever way you've chosen. In this how-to, I'm going to use the screw holes on the bottom tray of the Optima battery and a strap across the top because I'm poor and I had to roll 7 old ladies to get enough money for just the power wire.
9) With the battery mounted, go on and attach the two battery terminals to the battey. (P) for positve, (N) for negative. Now for the fuse holder, cut a length of 8-12" of the positive wire. This is going to be the wire that goes from the battery terminal to the fuse block. Cut the wire to a length you can use, but as short as possible. Strip both ends of it and attach one end to the fuse holder, and the other end to either a ring terminal or directly to the battery terminal.
10) Your fuse hold should be empty. It better be! GRR! Now, mount the fuse holder where ever you want, close to the battery though, because you want to minimize the length between battery and fuse holder, k? OK! There should still be a lot of slack power wire, cut off the excess and attach the wire to the other end of the fuse holder. Now you're hot all the way up; just without the fuse in place, so everything's still cold.
11) Now let's ground this bugger and go drivin! Find a good place to ground the battery in the trunk. Just like the front grounded distribution block, it needs to be as close as possible. In this how-to, my battery is in the spare tire well so I'll use the spare's mounting bracket as my ground point since it's right above the battery. Cut your wire to length and attach your last ring terminal to one end. Bolt that sucker down just like in the front-stripped paint, yadda. Attach the other end to the negative battery terminal and you're 'bout done buddy boy!
12) Go around and check every thing. Re-tighten every allen head screw in all the blocks and terminals. Re-tighten all the bolts for the two grounds. Check all the screws that hold stuff in place.
13) Lucky 13, put in your fuse. Go start 'er up, you're done cheif!
Thank agian, Ray.. your write ups are crystal clear, lamen terms, couldn't ask for anything better. 
.....my comcast cable internet has been givin me problems.....maybe thats it...sorry ray