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.
Starting at one end of rope, form a loose knot as shown; leave a 30-inch tail on right side and slack on left side. Gently pull down on loops.
Flip loops to left as shown. Lift the first loop and cross it over the second.
Using tail end, weave as shown.
Using slack end, weave diagonally toward bottom right. Pull rope through. Continue weaving back to top as shown, adjusting rope to resemble diagram.
Follow path of first round with slack end, staying parallel to previous round. Pull rope through after each pass, weaving until mat is formed.
Finish ends by using a needle to sew through the rope just below any frayed ends and tightly wrap thread around rope a couple of times. Sew through several more times to secure. Cut off end of rope just above thread. Singe frayed ends with a lighter to seal; sew to underside. Apply permanent craft glue to underside between ropes in spots to prevent shifting.