To create the return, you are going to make the opposite 45 degree cut that you made to the chair rail, with a 90 degree cut on the other side of that little piece. The 45 degree cut on the return will abut the chair rail, and the 90 degree cut will sit flush with the wall (or in this case, the side of my bookcase. A piece of lattice was run along that side of the bookcase to fill that gap you see above).
The little pie-shaped return piece is attached to the edge of the chair rail with wood glue.
After the glue sets up, fill any gaps between the return and the chair rail with wood filler. After that has dried, sand and paint the entire piece, and the seam between the two pieces will disappear.
Honestly now that the whole piece is finished, you can’t even see where the return meets the rest of the chair rail.
Way, wayyyyyy beyond my enjoyment of how these little corners turned out, is my sense of glee at learning something new. Never gets old, does it?!!