Replacing bad audio with a cleaner recording

As more and more videographers today are experimenting with the quickly advancing world of DSLR video, they quickly learn one rule—the recorded audio on DSLRs sucks! A true professional would almost never use their camera's built-in microphone, but most DSLRs have especially bad audio. While some DSLRs have microphone inputs, they still don't often have the best recording capabilities or feature sets, so many professionals use external audio recorders for their audio needs.

There are many out there, but a couple I highly recommend looking into are the Zoom H1 and H4n models (http://www.zoom.co.jp/?lang=en). They both record pristine audio quality in the WAV and MP3 formats to SD cards that can be imported instantly into FCPX. The H4n model also allows you to plug in XLR microphones directly into it.

So, once you've imported your video with bad audio and your separate high-quality audio file, what do you do? You used to have to spend a ridiculous...