It is a broad area and I'm not quite sure what you mean. Do you mean how do you know what number to put in for the looks of an item or actual wil editing?
Wil editing is best left if you don't understand how to get graphic numbers.
To get graphic numbers go to kaoris website. Download Koariis graphic editor and then load the items wil into it. The number for your db is that number minus 1.
If you don't understand pm me and I will write a better explanation.