The good message is, you do not need to know, how DMX works to use it.

This is, how to use it (basics):

First, I will describe it with a simple lamp (dimmer), later then I will show you the differences to a scanner. This way it is quite easy to understand.

A lamp can be dark, or bright or something in between, right? (You asked for an idiot's guide, so there it is ;-)
The first thing you have to learn is, that you can connect many dimmers to a single DMX cable. To tell the dimmer, how bright it's lamp should be, the dimmer needs a adress. This is a number between 1 and 512. The lighting console can say e.g.: "dimmer with the adress 234, you have the new intensity of 56%". Got it?

Ok, here is step two: If two dimmers have the same adress (lets say, 234) and the console says "dimmer 234, 56%", what will happen? BOTH dimmers will switch to 56%, because they do not know each other.

Usually, you do not use a single dimmer but dimmer packs. These have 3, 6 or 12 dimmers in a single case. But they only have a single adress. Why?
Well, the adress of the dimmer pack is the adress of the first dimmer. Lets say, you have a "six-pack", six dimmers in a single case. With the adress 234. Now, the first dimmer has the adress 234 (you told him), the second dimmer has the adress 235, the third 236 etc. We can say: "234 is the START ADRESS of the dimmer pack". Also we can say "the dimmer has (or needs) six channels". What happens, if you give 512 as start adress? Well, the first dimmer has the adress 512, all the other do not work. That is because 512 is the maximum adress of a DMX line.

Ok, now you know, what a start adress of a fixture (dimmer/scanner/whatever) is, and you know that a fixture sometimes needs more than one channel. This is the most importent thing to know about DMX.

Now, thing are getting more sophisticated. Lets say, you have a scanner that needs 12 channels (check manual). It has the start adress 1. And now you want to connect a second scanner to the same DMX line, what adress would you suggest. 13. because 1 (start adress of scanner 1) + 12 (number of channels of this scanner) = 13.

It can be a number higher than 13, thats no problem. But keep things simple.

I dont know GLP, but I think thats the way it goes:

Now you must tell the console (GLP?) that you have scanners attached to the DMX line. The console should have a database of all scanners. Somehow you can tell the console "I have scanner xyz2500 with start adress 1" (check console manual here). Now you can tell the console "I want scanner xyz2500 bright and green". The console will do the DMX-stuff for you.

Good luck.
Sebastian