There isn't really a specific number or anything you need to put into them.
Say you want to put a green light onto a wing (wink wink).
Say the plane is an F/A-18. I know that the Hornet has a wingspan of something like 14m.
If we look at images of the F/A-18, we also know that the green navigation lights are on right side of the aircraft; at the wingtip (where the leading edge meets the wingtip missile rail) and roughly halfway along the leading-edge root extensions (the things that come from near the nose and meet the front of the wing, in front of the engine intake).
Because the plane is sitting at the origin, each wingtip's X value will be 7 or -7
Now, our lights may be sitting in empty air; we therefore need to move it backwards or forward, by changing the Z position (eg, maybe Z= 1.5).
Maybe it's at the wingtip, but it's hovering under it. We might try a Y value of 0.5 then.
Now, it's really close, but it's not exactly in position, so we'll change the X value to 6.90, Y value to 0.45 and Z value to 1.53.
If the light is like a sphere or a 3D star, that you can see through the wing, then that's done. If it's only a little 2D light, you'll then have to go do the same thing for the top / bottom (the opposite side) of the wing.
Now that it's in place, we actually need to leave DNMViewer, because you cannot give lights the proper CLA in DNMViewer (they were introduced after DNMViewer was made).
This is where it gets kinda tricky, but not that hard (there isn't any maths involved
After you've finished your plane, open up Notepad (or the Mac or Linux equivalent) and drag your .dnm into the Notepad window.
The .dnm will appear as a bunch of numbers; don't stress. Open up the search function (I think it's under Edit -> Find) and search for the name of your light's .srf (exactly as you spelt it in DNMViewer).
The first result should be at the top of dozens of lines of numbers; don't worry about that one, go into the next result.
The next result should be near the bottom of the file, and it should have "CLA" and a number near it (next to it or under it, I can't remember). We need to change that number (which will be a zero if you didn't change it in DNMViewer), to the CLA number of a navigation light.
If we go to the CLA Tables thread here
on YSFHQ, we see that Nav lights are CLA 30; so we change the 0 to a 30.
Save (but do not "Save As...", otherwise you'll probably save it as a text document) and exit Notepad.
Open your plane up in YSF (once you have it in a .lst, and you have a .dat and .coll for it) and it should have the nav lights appear when you press (I think?) i.
In about a week when I get back home, I should have the opportunity (hopefully) to do another video tutorial. In that, I can go over some more stuff that people have confusion with (please let me know what I can include in these videos, or what I need to improve upon).
(EDIT: That was a close call; I clicked submit, but then it just refreshed and all my text was gone - maybe it was some kind of timeout Eric? Fortunately I have a Firefox addon specifically for recovering text written in boxes).