doc/script_commands.txt already explained how it work. Just try edit some existed scripts first, you'll learn by a bit each time.
I must say I'm able to coding because of private Ragnarok Online server too.
New update
https://raw.githubusercontent.com/kaninhot004/rAthena-item_db-to-itemInfo/main/easy_item_builder.txt
Easy Item Builder --- In case player curious which item to reduce skill cooldown just use this url and Ctrl + F then put skill name.
@MrColour_The solution is create new text file.
Create resourceName.txt -> Make your own resourceName list in this new file -> Remove my resourceName.txt -> Put your resourceName.txt -> Done.
---
Duplicate GvG map damage modiflier functions then create condition for check attacker was monster then do a multiplier there. (Try to use Find All in Visual Studio to find where to edit)
I will have a look in 2-3 days. As Rynbef said it's encoder issue.
You can try change in Notepad++ (Encoding, Language) try it many many ways maybe you will find a workaround.
if(countitem(.C_PetEggs[.@j]) > 0){
if(.@ctr == 10)
end;
delitem .C_PetEggs[.@j], 1;
.@ctr++;
}else end;
Store .@j index and delete it later because you need 10x amount.
Also nowaday rAthena already did the work for you.
No need my tutorial just use latest rAthena git. It's had Guild Emblem system embeded on it. (Make sure you changed IP on luafile)