Make sure you have a good battery first of all. The general rule of thumb is 1 farad per 1000 watts of power. Most of the brands out there are all fine, pick something that won't kill your wallet and doesn't have lots of blingy crap that you don't need. The more you pay for a cap, the more you've wasted paying for someone's marketing and advertising. Practically no one actually makes their own caps, they are all private labeled and there is a huge price swing for the same part from different brands. Don't bother with voltmeters, platinum terminals, and all that stupid shit, get something basic that does it's job.

Mount it as close to the amp as possible, with the shortest wire lengths that you can. An easy thing to do it just take your existing power and ground and run them to your cap, and then run short wires to the amp from there.

Also make sure that you have a large enough ground from the chassis to the battery, it's no use having big power cable and a bog ground back at the amp if the ground to the battery is undersized. You're trying to fix the weak links here.