sides that are long and 2 sides that are shorter. ignore doing any trimmings first.
to match the two ends of the frame, we had to cut the corner of the pieces. for this, we really had no measurements.
however, we tested it on some scrap board and got a shape to follow that fits.
don’t forget to include the inner piece of the cardboard (the one that supports the poster).
it will not be a straight diagonal line the whole piece through.
the only diagonal part if the front face of the frame.
trim the frame down wherever needed when doing this.
if you are not sure of how much to trim, always trim it larger 