It's usually pretty straight forward. I haven't dug into one yet but there is almost always a trim plate, in that corner on the inside of the door, to allow access to the mirror bolts. Some vehicles do require you to take off the door panel tho. Neither is something a novice can't handle just take your time searching for fasteners carefully, you don't want to damage the door panel itself. The OEM's have a habit of hiding some of the fastners well. Your fasteners will most likely be a mix of screws and plastic body clips, they look kinda like little plastic christmas trees. If you break a couple of those don't worry they're pretty easy to find in most cases and cheap. Most auto parts store like Napa, and O'Reilly etc don't really handle body parts like mirrors but they can get them usually. Just with a nice markup. Rock auto is a great tool if you can wait for shipping, their prices are generally untouchable from a retail parts store standpoint and they make it pretty easy to find the right part. If you want to get a peek at what it will take your local ford dealer should be able to give you a printout of that door panel assembly which will mark all the fasteners so you know where to look for them.
