Hi Scuby
Internal corners should really be scribed and not mitred- mitres will open up anyway as the timber shrinks when drying out. External corners are always mitred. I'm sure someone else may be able to post a link to a website that will give you instructions/ pics on how to accurately scribe an internal corner and you could try [broken link removed]. You'll need a mitre square, coping saw and a tenon or panel saw to do this. Basically you mitre the end of the board you wish to scribe, rub a pencil on the sawn profile to highlight the edge and then cut to this edge (at right angles) with the coping saw (for the curved profile) and the panel saw (for the straight section). A picture paints a thousand words- it's difficult to describe how to scribe in words alone! For a really good, tight fit the back edge of the scribed cut should be slightly undercut. On a well cut scribed joint it can be difficult to tell which board has been scribed.