The concept is based on Ohms law: the voltage across a resistor is equal to the resistance multiplied by the current through it. The board design in the schematic caters for a known 0.01 Ohm resistor with a 1% tolerance and a gain of 100 with a 1500 Hz low pass filter. The gain of 100 results in a voltage output that is proportional to the current: 1 A current gives 1 V output. This is so that we can use the oscilloscopes built in multiplication to see power in real-time. The low pass filter is there so that we don't get too much noise on the current measurement, but still enough response to see a spike when we start benchmarks.
I have posted images of the schematic and photos of the finished board. If you would like the Cadsoft Eagle design files, I'm happy to share them - just put a request in the comments below. Something I should also mention is the op-amp is pretty high end, unnecessarily! To be honest, it is a free sample that I gratefully received from Maxim, so I used it...
Input Current: ~50 mA - 5 A
Output Voltage: ~50 mV - 5 V (dependant on how close to the negative / ground rail the op-amp can go)
Frequency Response: -3 dB @ 1591 Hz
Supply Voltage: 5 V (dependant on the op-amp)
Power Loss in 0.01 Ohm Resistor @ 5 A: 0.25 W (one can use a spreadsheet to compensate for this error)