It is a little confusing in what he talks about. You just need to do it like he shows in the circuit without any extra diodes.
Here is what he is talking about:
- normally, with the LM317 , the lowest output it can do is about 1.2v. However, we want to be able to get down to 0v. One way to do this would be to use the LM317 and add a pair of diodes (and have a negative voltage supply) to shift this 1.2v down to 0v. See, each diode drops about 0.6v across it, so 2 diodes would be 1.2v. Basically, you are just shifting everything down by 1.2v (so if you had a design that went from 1.2v-5v, now would go 0v-3.8v). However, he mentions that using the 2 diodes, there is a problem with drifting. What happens is that the actual voltage drop across the diode changes with temperature. This would not be very good, as your reference voltage would change and then your output voltage would change. His circuit removes this problem.
- for power, all you need is the computer power supply. The computer power supply will convert your mains (110v-220v AC) to DC power. There are several DC voltages inside the power supply available (+12v, -12v, +5v, -5v, etc.). Take a look at the 2nd link that I gave you. You will use the +5v and -5v from the computer power supply to power the circuit.
The circuit has two adjustments. One adjusts the reference voltage (to set where 0v is) the other adjusts the output voltage (0v-3v).
You would then use a current meter and voltmeter to measure what is being applied to your electrode. I'll try to draw something up for you to make it clear.