But when I name the file _____zmymod.big it wont be read last, because of the "_____" it will be on top, right? And will the data from _gamedata.inc in the main folder be overwritten by my .big mod file when the modded file is the last one?
No, the "_______" characters are there because thats what will make the game read it last. And every file that is in the big that is loaded last (i.e. my example) will overright the original SUM2 files and the original edain mod files.
Also does the game know what data it needs to pull, and where it is located? So do I have to give the game a location to search for the _____zmymod.big file?
If the paths are correct then the game will know from where to pull the data ^^. The easiest method is to extract all the ini and inc files from the base game or mod (depending on what base you wanna mod) into a new folder (dont place this folder in the game directory! put it somewhere else like your desktop or something). this way the needed paths of the files are created correctly and you only have to drag&drop the modded files into the big (they must always start with data\... tho like in my example)
How to do this: Open desired big file you want to extract i.e. __edain_data. Then click edit -> extract all. then choose the destination and press ok and done. now all you have to do is mod your files you want in the newly created folders. when you are done drag&drop the modded files into your new big file (in my example the _____update.big). in the prompt message you press yes (you want to activate edit mode). then you should see something like this:
now you only have to shorten the path so that it starts with data\... then press ok. now your modded file is in your new big file and with the correct path.
when your modded big file is ready drop it in the main directory of your game (where you installed rise of the lich king. all the other edain files are located there too).
And my final question: When I want this mod to work on multiplayer, Is it that the only thing I need to do is give this file to my friend, or is there something else I must do?
yes that is correct your friend only needs your modded file. this is another beauty to this way of modding
. You only have to make sure that your friend has the same game and mod version like you but i think that is to be expected
.