Years ago I came across this simple little process that makes doing these types of joints fairly simple to achieve.
In this photo you can see that the panel section of this wall is actually wrapped around the end of the wall as well as the base kick being wrapped around.
1st pre-cut the parts in question, as you can see there are three parts that will get wrapped, each cut with a 45 degree bevel on the ends.
I place these parts face up with the points of the mitres touching, then using a clear packing tape I tape over the joint and the tape now acts as a hinge.
I flip the parts over as you see in the above picture, then apply glue to the inside of the joint and fold the whole assembly around the end of the wall, clamp in place and wait for the glue to dry.
Whala! a nice clean and solid joint, no nails , mortises, fasteners etc. Some sanding and ready for finishing.
The beauty of this system is that the tape dosen't care what angle you use.
Here you can see a 45 degree corner done in the same fashion.
A word of caution, Inside corners are a little more difficult due to the fact that the glue squeezes out of the joint as you fold it together so use minimal glue on a inside Miter.