Fairly easy to do and should take you about an hour or so if you do it carefully and clean along the way.
The back cover pretty much unclips by pulling the edges apart whilst the phone screen is 'facing' you. Few Torx screws (4 iirc) to unscrew and that should take the back of completely. You can then take it apart into 'two' pieces if you're careful, there's a ribbon cable that goes from the front half of the slide down to the bottom side so watch you don't pull it apart.
the front bit its held down by 4 screws under the slider. you can take the whole thing apart, clean it up, and then rehouse in an hour or two if you're extra careful.
Google for the N95 service manual.. it's got full instructions in there on how to do it all and what tobe careful of. Oh, and get a genuine Nokia housing.