Hello crapmemory
If the corners are true right angles ie 90 degrees internal and 270 external then it is not too difficult. I would recomend that you bought a mitre saw.The saw is fixed in a vertical position on pillars and the blade is then only required to be pushed and pulled. Provided you can measure correctly you should be OK. An ordinary hand saw would require you to use a mitre block. Make sure that you cut the mitre remembering which face of the dado goes against the wall! As well as glueing you may have to support the rail on small nails whilst the glue goes off. Gluing is Ok if the walls are true. Good luck