Rather than torment you with ascii schematics, let's talk our way through this. Connect the gate of a N-fet to the output of an opamp. At the source of this fet, connect a resistor to ground. Since the fet is a source follower, we have not done anything other than buffer the output of the opamp, which is driving a resistor. Tap the connection where the source of the fet intersects the resistor, and connect this to the negative input of the opamp. Connect the positive input of the opamp to a voltage reference of your choice. This voltage now appears across the resistor. The current in the resistor is V/R, and it flows into the drain of the N-fet.
If you need accurancy, note that offset of the opamp adds to the voltage reference. Perhaps a Max400.