There are much simpler ways to do this with add ons and other extra software, but the way it is intended to be done can be quite time consuming if you already have a large movie collection built in the Windows Media Center Library.
Every File has properties and in the file that you're movies in their are spaces for you to input data, (ie) Director, producer, year, synopsis. This will carry over and organize in WMC after you have opened your library, it make take a few minutes.
For cover art it is very simple but also time consuming, each movie file should be in its own subfolder within the folder you're collection is derived from, and in that folder a .jpg file should be inserted under the name "folder" (Lowercase) This .jpg will serve as the cover photo.
Windows assumes that users input this data naturally however that normally isn't the case.