I don't know if this helps, but I can think of three ways to handle the corner of the window:
1. Do a mitered corner (never my first choice);
2. Use a half-square triangle made in the two colors of the panes, placed in the corner where the two window panes meet;
3. Cut the window pane strips, and then cut a square in one fabric the width of the pane strips, draw a line across the back diagonally, place on the other strip and sew on the line, the way you do snowball blocks, trimming the excess fabric after.
Hope that helps!