Well, the honest answer is I don't I use what others have done before. A good
starting point is this site http://www.wikihow.com/Use-a-USB-Robotic-Arm-with-a-Raspberry-Pi-(Maplin), it gives most (pretty much all) of the answer, including where to get the pyusb library need; setting it up (be careful don't use the zip file but the tar.gz version - you might need to scroll down the screen to do this) and an example. A second source that was very useful was http://notbrainsurgery.livejournal.com/38622.html?view=93150#t93150 which contains an explanation of the 'triples' (for example [32,0,0]) used to select which part of the arm moves and what it does (in the example [32,0,0] causes the Robot Elbow to move down).
The code below is largely based on the code in http://www.wikihow.com/Use-a-USB-Robotic-Arm-with-a-Raspberry-Pi-(Maplin) the only real changes are named procedures (such as ElbowUp) in place of the slightly less easy to understand lines such as MoveArm(Duration,[4,0,0]) #wrist up . The only other changes was a procedure that changes what the light does (0 - light off; 1 - light on; 3 - pulses by a number of times).
See it action in the video below.
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