My sister in law made the subway art. She has it available for FREE at her website:
1) Measured it out.
2) Used my miter saw to cut 45 degree angles.
3) Used gorilla glue to hold it together. (I Don't love gorilla glue because is expands, but it is a strong hold & is what I had on hand)
4) Once dry I sanded down the Gorilla glue.
5) I caulked the places where the pieces of wood meet.
6) Sanded again once the caulk dried.
7) Painted the frame & let it dry.
8) And to keep a free project free, I backed the frame with cardboard.