if you import something which is published here on hive, it should be leakless (most of the time it should, I think if it isn't leakless and the author has no intention of fixing it, then mods take it down).
So all you have to is this:
1) copy all variables used by the spell - this is done either manually, or better yet, in the top bar of your WE, you go to File -> Preferences -> General -> tick (= cross) the field "Automatically create unknown variables when pasting new triggers" (or something in this sense)
2) Read "Import" instructions in the spell. If there is no such file (or trigger comment), then go to step 3
3) If there is a trigger which has all used variables listed in it, then select that trigger (in the left-side panel, where list of all triggers is), press CTRL+C to copy it, open your map -> open trigger editor -> select folder and press CTRL+V to paste it. Most of the time, such triggers are used to automatically create variables used by the spell
4) Repeat step #3 for all other triggers related to the spell
I'm not much to JASS myself, but I can't think of anything other that could be important and needed to be done.
However DO NOT confuse JASS with VJASS -> vjass is a "custom" language (a language based off JASS), but this needs custom program to run it - instead of your World Editor you use other program, but I dunno which it is, as I don't use vjass)