A basic very basic arm you can make from some things laying about.
2. Screwable pieces of toys or plastic.
3. batteries and battery compartment
First make the type of arm you want. From the screwable pieces. Hopefully it will work when you do it. The structure will move 360 degrees so make sure you make nothing too big.
attach the motor and battery compartment together then screw them so they don't break apart. My motor had plastic on it which i used with the screw.
This was a very basic Inscrutable it really is not any thing except a turning motor.