Draw State diagram of a locomotive with the controls given.

I hav no idea of drawing a state diagram ,can u help me pleas with this, The actual question is

Imagine a locomotive with the following controls.There are + and - buttons(each time presses,
they cause the speed to increase/decrease by 1), there is a lever with two positions Forward and
Backward (it can be moved only when the locomotive is at rest). There is also a bell that stays
on whenever the train is moving backward. There is also a red light thta stays whenever the
speed of atleast 90 miles per hour is maintained. Finally, there is a speed limit of 100 mph
forward and 20 mph backward(increasing speed has no effect). Using this,draw the state diagram