Who/how installs leather seat covers?

When dealers install aftermarket leather, they usually send the car out to a upholstery shop that specializes in doing this. You could probably find a shop to do this for you.

It would be possible to do this as a DIY project, but unless you are comfortable with this kind of thing, I would recommend getting a professional shop to do it for you. I removed part of the upholstery on my 07 to modify the seat height and reinstalled it successfully, but I did not remove the upper portion with the side air bag and did not remove the headrest. Removing the headrest inserts are challenging, even when you have the instructions on how to do it. BTW, brief instructions are available in the factory service manual in the Seat section.