I have an 09 Outback 2.5 XT Limited.

The cable for the fuel latch snapped suffer to corrosion. I bought the part but i want to install it myself to save $300.

It runs though the floor board starting on the drivers side floor and runs through the sub floor to the back passenger side.

 

I have no idea where to start however. Does anyone know how to do this?

Yep, you basically have to pull the rear seats out so you can get to the guide clips on the floor that hold the cable in place. You can probably pop the front and rear sill plates off and get to the cable where it runs along the left side by the drivers seat. From there it should run across the car under the rear seat, then turn and to back into the cargo area. Pull the plastic panels off on the right side and you'll see where it comes up from the floor and goes to the latch mechanism.

 

You'll want a trim clip tool and some screw drivers. A 12mm socket and ratchet for the bolts that hold the seat on in case you need to remove the seat back to get the plastic panel out.