It all depends on what model WRX you have!! some have Coils on top of the manifold and others have coils on top of the spark plugs (with should be held in with a 12mm bolt). either way you still need to check a manual for specs, gaps, etc....

Sorry, I'm not trying to put you off, its just I have seen too many people end up spending a s**t load more because the didn't follow the manual. It may sound silly as it only sound like a simple job (spark plugs, WHAT can go wrong) but believe me I have been working on Subarus since '87 and things do go pearshape!!!!