It's really easy. You don't even need to cut the power off but I always do. Never know when you drop that tool in there

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Go up in your attic and run a 20 amp rated wire down the wall and fish it through. Put a box in the wall for your new power outlet. Run the wire through your attic to your breaker entrance hole. I like to use a staple type clip to bind the wire to the studs going across. It keeps the attic clean and avoids tripping over it.
Run the wire down and take off the door and outer panel. At this time, flip the electric off. Look how the rest are done. you will have hopefully another slot available.
Split the wire up and run each type to it's correct spot. You should be able to tell easily by how the previous were done. Nuetrals go in a specific spot and so do the black. And the ground goes to a ground bar in there. Once it is all in place, pop your break on and then flip the power on.
I like to play my new circuit next to an existing one about 16 inches away. The reason is this. Some items you do NOT want on a GFI. I have one pump that is not on a GFI. That way if power goes out and flips the GFI which controls everything, when power comes on, that one pump will run. I do this with an external pump since it is not submerged.
It's really easy and the PITA is running the wire. Chances are, a neighbor will help you connect it since it takes all of 10 minutes tops.