It was super easy... We have willow growing out by the creek. I went and tried to cut the straightest ones. Cut them all to the same length and then weaved the twine at top and bottom.
I tried to tighten them pretty well, but didn't put too much effort into it. Took about an hour to do (In that time I also changed a diaper-and clothes, since he peed all over in the process! And Elle "helped" for a little while too... could have been done in about 40 minutes without kid-o's! =) ) Probably cost about 2 cents worth of twine!