Bundle the tops of the slender saplings together. You could tie them with cord, or even do a wrapping with your weaving vines. Next, start weaving your vines around the bundle, opening the bundle a little more as you go. A simple "over-under" weave is fine, but the resulting trap will be stronger if you "twine" two vines at the same time. This is done by twisting two vines around each of your sapling pieces. Make sure the gaps are minimal, or the fish will go wriggling out of your trap.