To end up with the same size as I made you'll need enough 1" thick lumber that is 26" long or longer to cut 15 strips, each one inch wide. 3 1"x6"26" boards should do it. You'll also need some jute twine to tie the thing together.
Let's get started!
Step 1: Cut all the wood to size
Step 2: Drill holes for the string
Swap out the drill bit for a 1/4" bit and use it enlarge the first 1/4" of the holes in the two pieces of wood that will be used for the outside edges of the doormat. The idea is just to countersink the hole so the knotted twine has a place to sit inside the finished doormat.