Although "12 pennies" idea will work. And qudos to him for taking the time to make a mock up for you.
It will be much faster and a cleaner look if you go with the compound cut.
The way I always did it is cut your two long boards first then connect them with the bottom short piece. That way you can sand the miters perfect. :thumbsup:
The way you always did it where? Go try it. :whistling2:
You're dealing with a compound miter. You'll lay the casing flat on the miter saw and set the miter to the angle of the wall, then set the compound cut to 45 degrees. If you don't have a compound miter saw you'll need to rent or borrow one.
12penny's method will let you do it without a compound cut and just a miter, but will look a little less professional (albeit more DIYer friendly). No offense 12penny...That is a great mockup and is a nice thing of you to do that!
seriously, go try it. :whistling2:
Sorry 12 but look at your piece #3 his railing has no horisontal spots on it except the bottom face. After he gets around the corner.
Solution= get yourself a compound miter saw or a contractor
I did tell him that his dimensions would be different than shown. Obviously the fit needs to be tweeked. This a DIY forum, without getting into the whole "what tools do you have thing" I offered advise anyone, regardless of his/her ability to pay $300 for a saw, could learn from.
And really, try to cut that joint. I've got two compound miter saws and neither of them can do it. The 45 compound miter cut is too long and will not mate to the short piece at the end of the wall. Something about changing planes. The only way to do it is to transition into it. If I'm missing something please post a picture. Never too old to learn.