The trim I bought was so thin that you could easily use a hand saw to cut the pieces.
I used a circular cutting saw because it was just sitting there and I’m all about making my life easier!
Cut two long pieces for each long side, the length of the base. Then cut two pieces for the short sides taking into account the size of the trim on the long sides.
The short pieces will fit inside the longer pieces.
All the measurements are in the post on my blog.