ok... so what you need to do first is have the wire that is coming directly from the battery(make sure there is a fuse on it as close to the battery as possible) connect that wire to the terminal on the amp that is labeled as "+12" or however it is labeled meaning power. -then depending on the amp it may have a terminal labeled REM. this would plug into the REM line in the back of the radio.- then there would be either a negative(-) or a ground(gnd) terminal. if it is a negative terminal, plug it into the negative line from the back of your radio deck. if it is a ground terminal, ground it to a major metal section of your car or to the ground wire of your radio. if you have two seperate terminals (one negative and one ground) you can just connect the terminals together with a little U shaped piece of wire and then run a wire from the negative terminal to the negative wire in your radio deck.
-an alternate place to hook up the power line (+12) other than directly to the battery is to the power line of your radio. you may have to get a bigger fuse since you will be drawing more power but if you hook it up to the switched power line of you radio deck it wont drain your battery
-any questions just ask. ive done it a ton of times. the best way to ask is to Prvt msg me.
~chris