First you write it in notepad, then you use Ctrl+C to copy it and Ctrl+V to paste it into a trigger. If you get error messages, try converting an empty GUI trigger to JASS(menu command) and adding JASS code to that.
Assuming no errors in the JASS code, it'll work fine.
Some JASS scripts require global variables and other things, which you'll have to set up beforehand.