Keep your cut string within arm's reach. Grab an unbroken bamboo leaf from the soaking vessel. Squeegee off the excess water by sliding the leaf through your index and middle fingers. Be careful not to give yourself a leaf cut, as the edges are surprisingly sharp and will definitely draw blood. Grip the ends of the clean bamboo leaf with both thumbs, as depicted, just outside the middle majority of the leaf where the ends begin to taper.
Using your dominant hand (we're right-handed) in the same gripping position, fold in the dominant end at the center of the leaf, creating a perpendicular edge where the respective end stem now points in toward you. The side furthest from you, where the perpendicular "edge" starts, is the pointy tip of the cone. Don't change your dominant hand's shape once in this position! Allow the back four fingers to gently round out and "cup" the back of the perpendicular leaf end. This hand is creating the cone hollow.
Keeping the shape of both your hands, fold the non-dominant side in over the dominant hand and now tilt the pointy tip down so that your dominant hand fingers are pointing straight down, into the tip of the cone. Still holding the leaf like so, spin your dominant hand toward your dominant side (counterclockwise for righties, clockwise for lefties) to tighten the cone until the pointy tip comes through with no hole.
That's it! You're ready for filling. Be careful not to loosen your non dominant hand, which should now be gripping the end of the cone like an ice cream cone. The tip risks expanding into a hole if you loosen or overly tighten your grip, and you'll have to readjust so that the rice mixture doesn't burst through.
Use a spoon to fill the cone gently, tamping down with a chopstick every now and then to get rid of air pockets and make sure it's tightly packed. Be careful not to poke too hard through your newly-created cone, or you might tear the bamboo leaf and have to start all over. Try not to adjust your non-dominant hand, so it better protects the original shape of the cone.
When the filling is roughly flush with the edges of the cone "mouth," grip the top flaps with your dominant hand.Fold the flap over the mouth, creating a triangle top. Gently lift just the thumb of your non-dominant hand to press down on the sealed top (above the tip facing you, we'll call this Tip A) as you use your dominant hand to fold the excess flap back onto itself, toward your dominant side.
Now you can move your non-dominant thumb, and you should be able to reach the excess flap with the fingers of your non-dominant hand. Squeeze all three sides of the top triangle to seal tightly. Grab a length of cut string and bite one end with your teeth. Thread the string down the side of Tip A opposite from you, wrap it under the cone and back up the side of Tip A adjacent to you, repeat the motion one more time, and finally knot it by releasing the free end from your mouth to tie a shoelace knot.