An easy project!
Three pieces of material are needed:
enough to make a back that is 18.75 x 35.35"
and a front that is 18.75 x 24.75"
a piece that is 18.75x8"
I had to improvise to get the back measurement, but that worked out okay. This is not rocket science.
Fold the trim strip in half and align it with the narrow edge of the top piece. Place the contrast piece under this with the right side up, aligned along the wrong side of the top. Pin and stitch through four layers.
Fold it down against the wrong side of the bottom and aligned with the front top.
Useful tools--seam ripper and small scissors for