So you want a programmable, setable, constant current source.
You need some form of analogue output, filtered PWM or a real D/A converter.
This needs to be fed into an op amp, which is monitoring the circuit current and controlling the transistor or FET that is producing the constant current.
While not too complex it is not trivial either.
This will not do negitave currents, it needs to be way more complex to do that.
Is this an assignment or do you just want something that works?