I recommend stitching
your left crest accent, then your right. Even if your first design moves
a bit from where it was intended, as long as you mirror it on the other
side, it will all look intentional. You may also find your designs
overlap slightly. That’s OK in some areas, but try to avoid stitching
satins over satins, as it often has a “splitting” effect on the threads
of your first design.
Finally, you can add that corner piece to the bottom, centering it
right in the middle below your heraldic figure.