Most spells from THW have instructions inside map that tell you how to copy spell correctly.
Although, if it doesn't contain instructions, then my advice would be to first go in File -> preferences and check box "Automaticly create unknown variables" or something like that. That way you won't need to worry about variables that the spell itself contains.
Afterwards, you should copy all the custom abilities, custom units and buffs if it has any.
Then go to triggers and copy paste triggers into your map.