Well, you probably should get permission from the creator/converter of the fish first, unless you plan to only keep it for yourself.
If you have other items installed, I suggest finding one that is in the position you want your item in (for example, if you wanted a hat copy the code of the Christmas hat) and copy the code. Rename it to whatever and try it out in-game, then just continue changing it slightly if it looks off.
If you don't even know what I'm talking out, go into the main FH folder and open items.cfg with notepad. It should be easy enough to figure out what everything is from there.