Let's begin with the trim itself. There are many different profiles of trim, and if you can't find what you're looking for I guarantee someone can make a shaper knife to cut it for you. Some trim is very difficult to work with and others, like straight non-profiled stock, are easy to deal with.
Our first step is determining the length of the piece. For our purposes let's assume we have a one foot section from inside wall to edge of the wall. How long is our piece? One foot right? Yes and No.
I will typically account for a little bit right at the end so that it doesn't come right to the edge of the wall. I believe this looks much better than hitting right at the edge. But, of course, that's just my personal preference. So we are saying one foot for our length. How do we cut this accurately though because the profile won't allow me to mark it perfectly on the outside of the half round circles that are there?
What I like to do is, if I have a straight cut piece on each end, is first figure the thickest part of the trim. This is the length that I will take off the end and mark on the backface of the trim. Let's assume the thickest part is 3/4". So I would measure 11 1/4" from the end and mark it for our 12" piece. I want the 45° angle to be from that 11 1/4" mark out away from the square end.
We are half done with it at this point. The second piece will be a very small piece and will be difficult to cut without a little thought. We'll want a longer piece than what we actually need which will make it much easier to cut. We want an opposing 45° angle to the angle we just cut. It doesn't matter how long it is at this point. After that angle is cut I will turn the piece upside down and right where the angle meets the backface of the trim (this will be against the fence) I will cut it straight off at that point.
Now we have the 2 pieces to finish up the miter return. But how do we make a nice joint? My suggestion is to glue the two pieces together before installing it. I use the blue painters tape and tape the pieces together (on the visible side only) first so the joint looks good. Then all I have to do to glue it is to open up the joint - the tape will hold it in the right position. Apply a thin layer of glue and fold them back together into the final placement. Apply another piece of blue tape to hold it there. You should only have to wait about 10 minutes for the glue to do it's job.
You can now take it and install it with just a few nails against the wall.
Below are several images that show the 2 pieces and how they would look installed.
Click for larger images