This prosthetic arm is an original design that uses strings to mimic the biceps and triceps flexing and extending. If you don't understand some of the instructions, look at the pictures to see how the design would work. Also, the colors are just for the functionality of explaining assembly.

The assembly instructions are below:

The first step is to use printer glue or hot glue to attach the orange claw pieces to the big black piece. Then, attach the strings to the claw piece as shown in the picture. The next step is to attach the small black piece with the orange pin. To prevent the pin from falling out, you can use a thin ring of hot glue on both sides. Finish attaching the string like in the picture. Now your very own prosthetic arm is functional and flexes and extends as the strings are pulled.