Basic layout of map file lines are:
L -X0, -Y0, Z0, -X1, -Y1, Z1, R, G, B
where, -X0 is starting X position reversed sign from in-game E/W direction (2nd number in in-game /loc), -Y0 is starting Y position reversed sign from in-game N/S direction (1st number in the in-game /loc), and Z0 is starting elevation (same as 3rd number in in-game /loc), X1, Y1, Z1, are similar but the ending positions, R, G, B are the 0-255 color values for the line.
P -X, -Y, Z, R, G, B, S, Text
where, X, Y, and Z are similar to X0, Y0, and Z0 above; R, G, B same as above, S is size of text from 1-3, and Text is the label
As an example, say some website gives the in-game location of an NPC named John Doe as +155, -411, +15, then to add a Large Dark Blue Label of his name, you'd put the following line in the appropriate map file:
P 411, -155, 15, 0, 0, 128, 3, John_Doe
Note how, the first sets of in-game nubmers get swapped, and their signs get reversed in the map file.
That's the basics, and all that most files have.