The voltage drop test is very simple. Use a digital DC voltmeter. You can get a pretty good one at Sears for $20 or less.

When you are doing a voltage drop test, you are looking at the voltage across each link in the chain. The total of all these will be the voltage across the entire circuit. For the starter circuit, start with one lead at the battery post and the other at the other end of the cable on the starter solenoid and have someone hit the key. If you have a fraction of a volt drop while the circuit is active then that means there is low resistance between the battery post and the solenoid post.

Next place one lead on the solenoid post and the other on the other big wire from the solenoid to the starter motor. Again there should be a very small voltage across this point when the key is turned. It is only a switch so if it is dropping voltage it has resistance, not a good thing for a switch.

Now put one lead on the big terminal of the starter motor and the other to ground and again hit the key. In this case you should see a high voltage (9 or more) because the motor is the load in the circuit. The cables and the solenoid were supposed to simply transfer current with low resistance, while the motor is supposed to use the energy, thus it will have high resistance thus a higher voltage drop.

Hope this helps.