Cut the inner tube to a length that fits around your neck comfortably
Put the nylon thru the plastic buckle and tie a not to hold it
Burn the cut edge of the nylon so it gets sealed
Put the buckle with the nylon inside the tube and leave a spare that you would use to flip and fold back after you tie a cable wire between the knot and the buckle. Do not forget to cut the wire
Pour the lead shots to the tube through a funnel, you can use a card wrapped as a funnel.
Seal the other end in the same way as before. Your neck weight is done! You can also tape it, I will add another step here when I tapedy thicker neck weight already.