An easy screw puzzle requires a minimal amount of assembly.
If you want to change the size, you can use the Customizer (under apps) to change the parameters of the model.

Assembly

1. Place the small screw part on top of the bolt (make sure the screw thread is the right way up)
2. turn over the nut and screw it on top of the bolt and small screw part

Puzzle
Now your task is to screw the nut back off (without breaking the part).