Gather the long grass into bundles about an inch thick. Lay these bundles to the side; they'll fall apart slightly because they're still loose. Lay them far enough apart that they don't mix together.
Place about 15 flexible, pliable branches vertically and parallel to each other on the ground. Line the ends up as best you can.
Gather the first bundle of grass together in the hands. Weave it over the leftmost branch, under the second branch, over the third and so on until you get to the last branch. Continue with the rest of your bundles, weaving the second over the first branch, the third under the first branch and so on.
Cut a 6-foot length of twine. Start at the bottom left corner of the mat and weave it over the first branch and under the second, weaving until you reach the other side of the mat. At the lower right corner, wrap the twine once around the first grass bundle end. Continue all the way around the mat. The twine keeps the mat together.
Tie the twine off at the lower left corner of your mat. Snip away any excess twine.