I want to do the same. There's a website that charges over a grand and they'll sell you an OEM nav stereo head thats pre-programmed for your vehicle (they need your VIN to do it). I was told there's a way to do it with Forscan as you can generally find the OEM nav units on Ebay for around $200. I'd rather go this route. If anyone has done this, links would be appreciated.
It is supposed to be mostly plug and play with the huge caveat of needing to install the external amplifier in order for the speakers to produce sound (the nav radio does not have a built in amplifier).
The forscan part can mostly be done trial and error style. With forscan you get to change parameters in the vehicle's modules to configure them. In the case of the nav radio for example, there is a parameter to tell it what brand of vehicle (ie ford, lincoln, mercury) it is installed in so that the appropriate startup logo can be displayed when the touch screen powers on. Some of these parameters are documented forscan and some are not.