Before you start, practice the braid by making a miniature version of the mat with thinner rope. When you're ready to make the big mat, cover a large work surface with kraft paper.
Draw a cross on the paper; it should have a 33-inch vertical line and a 12 1/4-inch horizontal line. (This is the dimension of the mat when you get to step 4.) As you weave, use the cross as a guide and tape down loops to retain the mat's shape.
We used 1/2-inch-thick twisted nylon rope. About 165 feet makes a 33-by-20-inch mat.