You may be better off asking specific questions.
What are the models of the sub and amp?
How big is the power wire that goes to the existing amp? Is the new amp going to be mounted near the existing one?
What headunit are you using? Does it have sub out rca connectors?
You may want to consider high passing the 6x9s, so they don't interfere with the sub i.e. they don't try to play the same frequencies. Better yet remove them, they are superflous with a sub.
What speakers do you have in the front of the car? This is the most important part of your sound system.
Anyway, first step is to mount the amp. Then run the power wire, from were will depend on the existing setup. Maybe you will need to run heavier wire to a distribution block them smaller ones to each amp. Or a second run from the battery. Make sure the fusing is correct for each run. We can advise on wire size and fuses when we know exactly what gear you have.