Cut the pork belly into half-inch-thick slices. Salt it.
Soak the rice and mung – separately – for a couple of hours – in water. After you drain the water, put half a teaspoon of salt in each and mix.
To prepare the bamboo leaves: Boil enough water to cover all the leaves. Add one tbsp baking soda. Turn off the heat. Add bamboo leaves. Soak for an hour. Rinse with clear water and soak in clear water overnight.
The next day:
Wash each leaf with a vegetable brush, length-wise on both sides. You’ll need three to four leaves per jung. Make a few extra in case of breakage or mistakes.
Cut the lap cheung in half lengthwise then half again width wise.
Take one bamboo leaf. Fold it in half width-wise so that the smooth part faces in. Then, lengthwise, fold about a quarter of the leaf up, to the seam, so that it creates a cone. Take another leaf and fold it around the first, with the bottom edge of the second touching the top leaf’s middle seam.
If you have Chinese soup spoons, use them to measure the ingredients. (Otherwise use a tablespoon.) Put a heaping spoonful of rice in the leaf, packing from the fold out. Add a heaping spoonful of mung beans. Add one piece of pork belly in the middle, one piece of lap cheung on top. Cover with another heaping spoonful of mung then another heaping spoonful of rice on top.
Take a third leaf; wrap it around the second, with the bottom edge of the leaf running along the seam of the second. Pat down the ingredients and hold the cone firm while folding the third leaf lengthwise and closing it on the same side that the first leaf’s edges are folded up. Grasp the jung from the bottom, keeping both edges closed. Fold the top edge of the leaves down on the same side as the other closures. Smooth the leaves out. Hold it firmly, but don’t squeeze it too hard; it will break the leaves and make everything fall out.
Hold the end of the string with your teeth. Wind the other end around the jung width-wise maybe 10 times, so that you have sealed the whole length of the jung. Criss-cross the ends of the string as if you’re tying a package. Then wind the string twice around the jung length-wise. Make a double-knot.