While you're waiting for the book (also available from TRF), here's a few basic steps.
Turn on the headlight switch, so there is a load on the system (or would be if you didn't have a problem).
Start by touching the test lamp leads to the battery clamps. You should get a nice bright light.
Now one lead to the positive clamp, the other to the engine block. Should see same brightness as before.
Hot clamp to a good body ground. One of your leads should have a point, which you may need to use to dig through any paint or rust.
Now connect one lead (usually with an alligator clip) to the negative clamp (or another convenient ground). Double check that you still get a light when probing the battery hot post/clamp, then check the hot post on the starter.
From there, it depends a bit on year as Ken said. Should be one or more heavy brown wires connected to either the starter post, or the connector in the cable that I mentioned before. The Advance Auto Wire site has original schematics for each year (which are also in the book). Probe the brown wires(s) to check for power there.
On years with an ammeter, also check the output terminal at the alternator. If no juice there, your ammeter may have failed (or the wires to it).
If you still don't find a problem, you'll need to get access to the ignition switch and headlight switch. Check the brown or brown/white or brown/blue wires at the switches for power.
With the headlight switch still on, it should pass power to the blue wire. If you need to still work towards the headlights, check the wires at the high/low beam switch. It should have power coming in in the blue wire, going out on either UW or UR.
Back at the ignition switch, you can turn the switch to 'Run' and check for power in the white wire.
Etc, etc. Eventually, you'll come to a place that should have power and doesn't, meaning the problem lies in the section you just moved past.
Or, when working on the ground side, you'll come to a place that is hot and shouldn't be. Again, you just moved past the problem.
Once you find a problem, stop testing and fix that problem. Then check for symptoms again and, if necessary, restart the process of tracing along the circuit. Although usually there is only a single issue that you just fixed, sometimes it happens that multiple issues show up at the same time, or even that a new issue appeared while fixing the last one.
Also a good idea to keep an eye on those headlights, in case the problem vanishes temporarily. Moving things around will often get a bad connection to conduct for a time. In that case, wiggle and move wires in the area you were working on, to see if you can get the problem to come back.
PS Don't forget to turn the switch off if you walk away. Even if the lights don't work, there may be enough drain through the switch to run the battery down. Or the gremlins may make the lights work while you're not watching, just to frustrate you (and kill the battery).