You just have to remove the parts of the wall or add to them to make the vent fit. That is why it is retro fitting as you just have to make it fit in the space.
These can be tricky. Just remove the old vent and measure the interior diameter of the hole uncovered. Then make sure to measure the "male" section of the soffit vent and make sure it is smaller than the hole and you'll be fine.
To install a soffit vent in your home you need to cut holes the size of your vent into the soffit. The vents are then screwed into place. These go between the rafters.
